could it be a type of spounge?
we need more of a discription. Or better yet, a picture.
for what you said it sounds anything from a spounge to the crap that builds up on your raw shrimp when cycling your tank.
do you have a cleanup crew? If the tank is cycled then put in a cleanup crew and they will take care of that algae, I don't know the name of that algae but it is normal most tanks have it and the cleanup crew takes care of it.