A picture of it would be helpful. Based on color, it could be black bearded algae. All algae is caused by an imbalance in the aquarium between the three things that plants (algae included) need: a light source, a carbon source and 'other nutruients' source (aka fish waste, plant fertilzers, decaying food).
Do you have live plants in the tank? If so, what kind? A lot of plants sold in aquarium stores are not aquatic and will decay over time in the tank, adding to the excess 'other nutrients' in the tank.
