Your tank is still very new, and I'm pretty sure that what you're seeing is probably the brown diatom algae phase that all of us went through with new tanks. The stuff is usually worse after the lights have been on for several hours, and not as bad in the morning when the lights first come on? It pretty much has to run its course and use up the silicates in a new tank (from the rock and substrate) and eventually it should disappear.