This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Forms in soft brown clumpy patches. In the freshwater aquarium, these
are usually diatoms. Usually indicates a lack of light or an excess of
silicates. Increased light levels will usually make it disappear.
Easily removed by wiping the glass or siphon vacuuming the affected