Water, clarity and algae
Mustard algae in a pool
Mustard algae looks like sand or pollen on the shady side of the pool. It brushes off easily — and settles back just as easily.
Mustard algae is a yellow-brown algae that collects in shaded areas, on steps and in corners. Unlike green algae it rarely clouds the water, which is why it is often mistaken for sand or dust. The giveaway is that it brushes into a cloud and then resettles in the same place within a day or two.
Water guides explain what a reading or a symptom means and the order in which to correct it.
It is resistant to chlorine levels that clear other algae, and it clings to swimwear, inflatables, pool toys, brushes and robotic cleaners. Those items carry it back into the pool after treatment, so a lasting result depends on treating everything that has been in the water as well as the water itself.
The short version
It resettles after brushing
Sand stays put when brushed; mustard algae disperses and comes back to the same shady spot.
It tolerates normal chlorine
Clearing it needs a higher level held for longer than routine dosing provides.
It hitches a ride
Swimwear, toys and cleaning tools reintroduce it unless they are cleaned too.
How mustard algae is cleared
Brushing every affected area
Brushing lifts the algae into the water where chlorine and the filter can reach it, and is repeated daily during treatment.
Raised chlorine, held
Chlorine is raised well above normal and kept there, with pH in range, until no algae returns after brushing.
Cleaning the filter and tools
The filter is cleaned or backwashed thoroughly, and brushes, nets, toys and robotic cleaners are washed in chlorinated water.
To stop it coming back
Wash swimwear and towels
Algae can survive on fabric and return with the next swim.
Treat the cleaner
Robotic cleaners and their filter bags should be cleaned and dried after treatment.
Improve circulation in shaded areas
Adjusting return jets towards the shady side helps prevent resettling.
Common questions
Is mustard algae dangerous?
It is not usually harmful in itself, but it can shelter bacteria and should be cleared rather than ignored.
Why does it keep coming back after treatment?
Most often because swimwear, toys or cleaning equipment carried it back into the pool.
Does an algaecide help?
Some products formulated for mustard algae help alongside raised chlorine and brushing.
Related pool care guides
- Black algae in a poolAnother water problem with a related cause.
- A pool that turned green while you were awayAnother water problem with a related cause.
- Treating a green pool or draining itA related choice about how the pool is looked after.
- Restarting a pool that has been left unusedPool care for a related situation.
This guide sits under green pool recovery. Browse all pool care guides. We look after pools at homes and commercial properties across Dubai.
Ask about your pool
A clear photograph and your latest test reading are usually enough for a useful first answer.
Phone
+971 56 181 0106Talk through what the pool is doing and book a time for us to see it.
WhatsApp
Send a messageThe easiest way to send photographs of the pool and plant room.
Next step
Seeing yellow dust that keeps returning?
Send a close-up photo of the affected area. We will confirm whether it is mustard algae and plan the treatment.