This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Grows in bright green clumps in the gravel, around the base of plants
like Echinodorus and around mechanical objects. It has a coarser
texture than "beard algae". Beard algae will ripple in the water
current, hair algae tends to form matted clumps. Individual strands
can get to 5 cm or more. This is easy to remove mechanically by
twirling a toothbrush in it. Can be troublesome if left unchecked.
This is a popular food supplement for fish among European aquarists.