Mustard Algae in Your Pool in Los Angeles: What Works

Mustard algae, also called yellow algae, is a form of green algae that grows as a dull yellow-brown film on the surface. It tolerates ordinary chlorine levels, which is why a pool with a normal reading can still have it. It rarely clouds the water. It settles on the wall that gets the least sun and sits there looking like sand or dirt.

People usually notice it after brushing. The yellow lifts off in a cloud, the wall looks clean, and by the next morning the same patch is back. That return is the giveaway.

Is it mustard algae or pollen?

Yellow-brown dust on a pool has two common causes, and only one of them needs treatment. Oak and pine pollen and ordinary dirt settle everywhere. Mustard algae picks a place and stays in it.

• Where it sits. Algae favours shaded walls, the north side, steps and the bottom of the shady end. Pollen and dirt lie on the floor and on the water in every part of the pool.
• How fast it returns. Brush it and run the filter for a night. Pollen is gone, caught by the filter. Mustard algae is back in the same patch within a day or two.
• What chlorine does. If you add a normal dose and the patch does not change, it is not pollen.
• Where else it shows up. Look at the ladder, the underside of a float and a swimsuit left in the pool. Yellow smears there mean algae.

Why LA pools get it

Mustard algae needs a surface, shade and low-flow water. Most Los Angeles yards supply all three somewhere, even when the rest of the pool is perfect.

• A tree on one side. A jacaranda or an oak throws shade on one wall all day. That wall is where the film starts.
• Stabilizer too high. Cyanuric acid makes chlorine last in the sun, but past about 80 it slows the chlorine so much that a reading of 3 ppm acts like 1. The stabilizer guide explains how to check and lower it.
• Dead corners. Steps, behind the ladder and around the light get little circulation. The film starts in the corner nobody brushes.
• Things that travel. Swimsuits, floats and toys that spent time in another pool or a lake bring spores in. Algae also rides on a net or a brush used elsewhere.

How to get rid of mustard algae, in order

Plan on three to five days. Keep swimmers out of the water until chlorine is back in the normal range.

1. Test and write down the numbers. Free chlorine, pH and stabilizer. The stabilizer reading decides how much chlorine you need.
2. Bring pH to 7.2–7.4. Chlorine works much faster at the low end of the range.
3. Brush every yellow patch, walls, steps and floor, before you add anything. A nylon brush is enough: mustard sits on the surface and does not root into it. Brushing lifts it into the water where chlorine can reach it.
4. Shock in the evening and hold it. Raise free chlorine to about 40% of your stabilizer reading, so 12 ppm at 30 CYA, and keep it there for two or three days. Test twice a day and top up when it drops. A dose that is gone by morning leaves the algae alive.
5. Treat everything that was in the water. Ladder rails, floats, toys, the net and the brush go into the pool during the high-chlorine period so they are treated along with the water. Swimsuits and towels are washed in hot water with chlorine bleach or an oxygen cleaner. If you skip this step, the algae comes back on them.
6. Brush again every day. Once the yellow is gone from the walls, brush once more to be sure.
7. Add a mustard algae treatment if patches survive. Products sold as yellow or mustard algaecide work best after the shock, not instead of it. Use the label dose.
8. Clean the filter and run it around the clock. Dead algae collects in the cartridge or the sand. A dirty filter returns it to the water. Hose a cartridge, or backwash a sand or DE filter, at the start and again at the end.

Is it clear? Brush the shaded wall once more the next day. If nothing yellow comes off and the chlorine reading holds overnight, it is dead. Dead mustard algae looks like grey-white powder or just disappears into the filter.

Why the first shock often fails

The common mistake is treating mustard algae as green algae. Green algae floats and goes with one shock and a clean filter. Mustard clings, shrugs off a normal dose, and is hiding on your gear. If the pool is green and yellow at the same time, clear the green water first with the green pool clean-up guide, then work through the list above.

Keeping it from coming back

• Brush the shaded wall every week, plus steps and the area behind the ladder. Algae that is knocked off weekly never settles in.
• Keep chlorine steady, not up and down. A day at zero is enough in summer.
• Check stabilizer monthly. Above 80, stop using tablets and plan a partial replacement.
• Rinse and dry gear that has been elsewhere before it goes into your pool.
• Watch the phosphate reading in a pool that keeps growing algae. The phosphates article covers when it matters and when it does not.

What it costs to have it treated

A weekly visit already includes brushing the walls and checking chlorine, from $159 a month. If the yellow has taken hold, we quote the clean-up on its own: it means a few extra trips in the same week to brush and redose. Our recovery without draining is $349–$749 for a home pool. Both numbers are on the price list. If the water is also green, the green-to-clean service covers both.

Not sure if it is algae or pollen? Send a photo of the patch and say whether it came back after brushing, or call (818) 740-6099.