This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Grows on plant leaves and is bright green. Individual strands have a
very fine texture but it grows in thick patches and looks just like a
green beard. It grows up to 4 cm. It cannot be removed mechanically.
This does not indicate bad water quality but grows very fast and
overtakes the tank, making it a "bad" alga. Can be eliminated with
Simazine (Aquarium Pharmaceuticals "Algae-Destroyer").