I second diatoms on that. how old is the tank? I would check your phosphates and nitrates and make sure that they are in check. If the tank is fairly new it could be the initial algae breakout that you are dealing with. How long are your lights on for?
Its a pretty natural part of a cycle, but I would say you should have your lights on for 12 hours or less, if at all. If you are just cycling with no livestock in the tank might be best to keep the lights off.
With no corals in my tank I have my lights on 10k lights from 530-11:30 and antics from 630-1230. Figure that my fish don't need that much lights I run them for that long just so when I do start adding corals that I don't get a major algae breakout.