How to get rid of mustard algae in a pool

Mustard algae, also called yellow algae, is a chlorine resistant algae that grows as a yellow, yellow green or brownish film on walls and floors, usually on the shady side of the pool. It feels slimy rather than rough, and unlike black algae it does not root into the plaster. It just keeps coming back.

Pollen floats, mustard algae sticks. Every spring in Palm Beach County, oak and pine pollen covers pools in a yellow dust. Pollen sits on the surface, collects at the waterline and in the skimmer, and wipes away. Mustard algae clings to the walls and floor, usually in the shade, and returns in the same spots after you brush it off.

If you brush a yellow patch and it is back in the same place two days later, it is algae.

Low chlorine, shade and poor circulation let it grow, and gear carries it in. Swimsuits, floats, toys and even pool brushes used in another pool, a lake or the ocean bring it along. Once it is in, it hides in the filter, on the tools and on anything that sat in the water.

Chlorine held high for days kills it, but only if you clean everything it can hide on and brush first. The order matters, because the reason it comes back is almost always something that did not get treated.

• Treat the gear. Wash swimsuits, and put toys, floats, brushes and the skimmer net in the pool during the shock.
• Brush the walls and floor to stir it up into the water where the chlorine can reach it.
• Lower the pH to about 7.2 and shock heavily after dark. Hold the chlorine high for at least a day.
• Run the filter nonstop and clean it once the dead algae has been caught, because the filter is a favorite hiding place.

As always, the CDC rule applies: chemicals go in one at a time, never mixed.

Not during treatment, and ideally not until it is gone. The chlorine will be far above swimming level, and algae in the water means the chlorine is losing. Wait until free chlorine is back to 5 ppm or lower. See how long after shocking can you swim.

Usually a few days of treatment. Check the shady walls a week later. If it comes back in the same spots, something reseeded it, usually the filter or a piece of gear that missed the shock. If the whole pool has gone green by then, a green pool cleanup is the faster way back.

Keep chlorine steady, brush the shady walls every week, and keep gear clean.

• Steady chlorine of at least 1 ppm, or 2 ppm with stabilizer, per the CDC.
• Brush the walls and steps weekly, the shady side first.
• Rinse floats and toys used anywhere else before they go in.
• Keep the filter clean. See how often to clean a pool filter.

Brushing the walls and steps is part of every weekly pool service visit.