Does it pull off quite easy and feel quite course and dry instead of wet and slimey?
Its a kind of algae. If its the one i think it is then it isn't too bad. It grows quickly but tends to stay in one spot and doesnt really stick to anything so really easy to remove. Not sure what it is called but I get it in my tank and it only grows in front of my spray bar. Only takes a couple minutes a week to pull it all out and in some spots it actually looks quite nice (my baby shrimp and fry love it).