Last Updated: November 14, 2021

How to Identify, Prevent, and Remove Mustard Algae

Written By: Pool Care Guide

Mustard Algae, also known as yellow algae or green slime, is a type of aquatic microalgae that can be found in pools. It thrives in warm water and can proliferate on surfaces like filters, sides of pool walls, and steps where it may smother other organisms. Mustard Algae is easily identifiable with its bright mustard-yellow coloration; however, you should look for signs that this alga is present before using chemical treatments to get rid of it.

Where Does Mustard Algae Originate?

Mustard Algae is a form of green algae. It is chlorine-resistant and will take a fair amount of scrubbing to kill. Mustard algae live in both marine and freshwater but thrive best in warm temperatures. Because it can grow rapidly on pool surfaces, mustard algae make their home primarily around swimming pools.

How to Identify Mustard Algae

Mustard alga's coloration may vary, but it is typically bright yellow and green. Sometimes you will find a mixture of these colors on an algae strand. Mustard algae can grow up to two inches long!

The presence of mustard alga, if left alone for too long, can completely coat your pool wall with slimy orange stains. Not only does this turn your water green and make it difficult to see through (making the use of chemicals more dangerous), but it also makes steps slippery.

What Does Mustard Algae Do in a Pool?

The presence of mustard algae can cause all kinds of pool problems. First, it makes the pool look dirty even after you have cleaned it. It also decreases the efficacy of your sanitizer and filters by covering them in a slime that is hard to remove. If left alone for too long, this alga will cover your entire swimming pool with a coating of slime.

How to Prevent Mustard Algae?

There are many ways you can prevent mustard algae from growing in your pool. You can use ultraviolet sterilizers or metal-removing algaecides to kill any existing algae. If you want to purge the mustard algae from your pool thoroughly, you'll need to take a hands-on approach. Here's a list of everything you'll need to do:

Step 1: Balance Your Pool Chemicals

Since this alga thrives in warm water, it is important to balance your chemicals before getting started. Pay extra attention to your chlorine, pH, and alkalinity.

You need the chlorine to sanitize, the pH to keep the chlorine effective, and an appropriate amount of alkalinity to prevent sanitizer losses from chlorine evaporation.

Step 2: Keep Your Water Circulating

Once you have balanced your chemicals, it is important to keep the water circulating. This will help prevent any algae that might be present from settling on surfaces in your pool.

Step 3: Keep Your Pool Clean

The best way to keep your pool clean is by vacuuming and brushing its surfaces regularly. If you are able, remove the debris from the water before allowing it to settle on surfaces in your pool.

If you don't believe you are likely to keep up with the scrubbing and vacuuming, consider purchasing a robotic pool cleaner. It won't scrub the walls to the same degree as hands-on work. However, it will help to keep algae particles from securing themselves to your pool's surfaces.

Step 4: Sufficient Sanitation Practices

Use bleach-based cleaners to sanitize covers, ladders, slides, and other pool equipment and gear. Also, make a pool rule that all swimmers must take a shower to rinse off any potential algae picked up from another body of water.

How to Remove Mustard Algae from Your Pool

If you have mustard algae in your pool, it is important to remove the alga as quickly as possible. The longer it lives, the more likely its spread will be difficult to stop.

Step 1: Scrub the Pool

Start by brushing any visible strands of this slime off walls and into the water, where they can be easily vacuumed up or skimmed away with a net.

You should also scrub the surfaces of your pool with a chlorine-based cleaner and a stiff brush or scouring pad. If you have a robotic pool cleaner, run it through an entire cycle to make sure all parts of the machine are rinsed clean as well!

Step 2: Vacuum the Pool with the "Waste" Setting

Vacuum your pool with a hose attachment to remove any remaining particles once you have brushed away as much of the mustard algae as possible. Be sure to use the "waste" setting to help capture any extra debris and prevent it from resettling in your pool.

Step 3: Balance Your Pool Chemicals

If you have followed the steps above, your pool should be relatively clean. You will need to balance your chemicals before adding any sanitizers or other treatments for this alga's removal.

Step 4: Brush Your Pool Again

Once your pool chemicals have been balanced, brush the walls and floor again to remove any remaining strands of algae.

Step 5: Place Hoses and Maintenance Equipment into Shallow End of Pool

After you have done all that can be done to clean your pool, place any hoses or other equipment into the shallow end of the pool. In the next step, you will be performing a double-shock. Since the chlorine levels are high, we can use that sanitizing power to cleanse your pool equipment of any algae that could reinfest your pool.

Step 6: Perform a Double-Shock or a Quad-Shock

Use a double dose of a standard pool shock to raise your pool’s free chlorine levels to at least 5 PPM. Without a stabilizer, chlorine evaporates in minutes when in direct sunlight. With that in mind, start the pool shock at sunset.

If you want to ensure that you'll kill the algae, you can increase the double-shock to a quad-shock. In that case, raise the chlorine to over 10 PPM. This shouldn't be necessary most of the time. Quad-shocks are usually reserved for blue-green algae, the most difficult algae to remove.

Steps 7: Purge Algae from Your Pool Filters

You're at the halfway point. Before continuing, you should clean your filters through a backwash or by switching your filters to "waste." We need to get rid of any algae that might be hiding away inside the filters.

Step 8: Balance Your Pool Chemicals and Scrub the Pool

Once the filters are cleaned, balance your pool chemicals and scrub away any remaining visible algae. Your pool should be close to being clean of mustard algae!

Step 9: Perform One More Pool Shock

Finally, complete the algae removal with a final pool shock treatment. Don't forget, sunlight is bad, so perform it at night.

Questions and Answers

What Is the Best Algaecide for Mustard Algae?

Some algaecides are designed to treat blue-green algae, which is different than mustard. If you have mustard algae, Polyquat Algaecide is a spendy but recommended option. Metallic algaecides are also very effective, but they tend to stain pools. The product is safe for use on vinyl liner pools and can be safely used with other pool chemicals. 

How Do I Prevent Mustard Algae from Coming Back?

The best way to prevent a return of this stubborn alga is discipline. Create a maintenance schedule that contains the following list of to-do items:

  • Periodically change and clean filters to keep the algae from finding a hidden safe harbor point where it can grow and shed particles to the rest of the pool
  • Scrub any algae that you see
  • Remove all debris
  • Balance chemicals
  • Periodic algaecide treatments

How Much Algae Is Too Much?

As a general rule, you should treat and scrub any algae you see. However, if you want to push it off a bit, you can wait until there are about 20 spots that measure at least one inch in diameter. Each spot is likely home to thousands of cells that can quickly reproduce and spread throughout your pool if left untreated.

What Type of Algae Is Mustard?

It is important to note that mustard algae look similar to many other types of algae. To be sure, you should purchase a testing kit from your local pool store or online retailer.

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