This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
This grows in feathery black tufts 2-3 mm long and tends to collect on
slower growing leaves like Anubias, some Echinodorus and other wide
leaf plants. Also tends to collect on mechanical equipment. This is
actually a red alga in the genus Audouinella (other names:
Acrochaetium, Rhodochorton, Chantransia).
It cannot easily be removed mechanically. Remove and discard the
affected leaves. Equipment can be soaked in a 25% bleach solution,
then scrubbed to remove the dead algae. Siamese Algae Eaters
(Crossocheilus siamensis) are known to eat this algae and can keep it
in check. A more drastic measure is treatment with copper.