Sounds like it could be cyano and/or diatoms. I assume your tank is fairly new.? Using good source water, and limiting feedings will help control the algae and/or diatoms. Cutting your lights out for a couple days, and then running them on a decreased photoperiod is recommended. Using a chemical media like GAC, Rowaphos, etc should help also. Manual removal of the algae is about the only option. Doing a PWC after cleaning and running some mechanical filtration is a good idea to aid in removal. Be sure to clean any mechanical filtration every few days. HTH