This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Looks like individual strands of hair algae but tends to grow in
single branching strands like a deer antler and is grey-green. Seems
to grow mostly on tank equipment near the surface. Difficult to remove
mechanically. Soak affected equipment in a 25% solution of household
bleach and water to remove it.