Is it a newer tank? If it's brown algae (diatoms) it's common in virtually all new tanks. Trust me, I'm dealing with it now. As mommytron said, it's normally fueled by the silicates leeching out of new substrate, decorations, etc..., as well as the water conditions caused while your tank is cycling Its supposed to go away as quickly as it came once it's used up the silica in the tank. If it's caused by silicates actually found in your tap water (there's a test kit for this), you've either gotta start using RO water or running a resin in your filter that will remove them. I'd just give it a few weeks or a couple months and see if it goes away on it's own.