How old is the tank and where are you getting your water? Also what kind of lights/bulbs are you running? Possible that is contributing, if the bulbs are old/wrong color spectrum. Obviously I would expect it to be algae of some type, but wondering what the source/cause it, versus treating the symptoms. Running any carbon? That can help clean your water also.

This thread might be better in the emergency section. Some tank/set up parameters would be helpful, also maybe a photo. Maybe a mod can move it.