This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Grows in thin, hard, circular, bright green spots, usually on the
aquarium glass but also on plants under high light conditions.
Considered normal for planted tanks. Must be mechanically removed. On
acrylic aquariums, use a cloth pad or a gentle scouring pad like a
cosmetic "Buff-Puff" and a lot of elbow grease. On glass tanks,
scraping with a razor blade is most effective.