Here's something that Ben wrote once. "... to firmly determine what the spots are, swim down, and scrap the spot with your fingernail. If the spot IS black algae, you'll end up with a bit under your fingernail. Swim up, and smear the algae on a bit of white cardboard or the unlined side of an index card. Black algae will look almost black, but will smear dark green."

I would recommend running this test first.