I am assuming this is hair algae, is it fairly long and like hair waving in the water?
Need some more info: water perameters, incl, Ph, Alk, Calcium, Ammonia, Nitrate, Nitrite, phosphate.
Do you use RO/DI water or distilled water? If not you should tap water has phosphates and silicates in the water and just feeds the algae.
Also would help to know what is in your tank, size, filter, cc or sand bed. as much info as possible to help you with your problem. Also do a search on hair algae there are tons of threads with great info on them.