Welcome to AA btw! Sounds like diatoms which feed off of silicates which can come from your sand and/or your water source. What chiclidlover was talking about is usually the source of something like a hair algae bloom. Anything that's not RO
water like RO
, distilled, or tap has dissolved solids like metals and such which contribute to something like hair algae, but not necessarily diatoms. How old is your tank? Almost all new tanks go through a diatom bloom and sometimes it comes back (even if you used RO
from the beginning).
If you'd like to ID the type here's a guide:
Nuisance Algae ID Guide
Don't worry, the only negative about this really is it's not appealing. Getting RO
water or at the least distilled if you aren't using it would help. Also not stirring up the sand(don't know if this is something you do/don't do) will also help get rid of them. They usually starve themselves and die until there is a build-up and come back again.