Mustard Algae
Mustard algae is a yellow-brown pool algae species that brushes off easily but quickly returns, caused by a chlorine-resistant strain.
In Plain Language
Mustard algae (Phaeophyta variants) deposits as a powdery yellow or mustard-coloured substance on pool walls and floors, particularly in shaded areas. It brushes off easily and can be mistaken for sand or pollen. However, it re-establishes within days without a sustained high-chlorine treatment. It is resistant to normal chlorine levels but is effectively treated with a thorough superchlorination of 30 ppm FC and simultaneous treatment of all equipment that contacted the water.
Why It Matters
Mustard algae is often misdiagnosed as dirt or sand, leading to repeated ineffective treatments. Recognising it allows targeted shock treatment.
Typical Values
Last reviewed: 2026-06-01