sounds like an algae bloom. Has the tank finished cycling? Do you have real plants and how much light gets to the tank. Algae is caused by nitrate which is the end product of a cycle. Real plants use nitrate as food, but if you don't have any, alga will flourish on it also. If your tank has finished cycling, how about adding some algae eaters? BTW - what size is your tank and what are the water parameters, ie. ammonia, nitrite, ph etc.