A picture would help see if it's truly brown algae or not. Brown algae is due to a
diatom bloom in the tank, usually seen in new tanks as diatoms bind all of the free silicates in the sand, rocks, and water. It will pass on it's own, but there are things you can do to speed up this process. First, make sure that the water you add to the tank is as pure as possible - ro/di water is ideal. Second, you can siphon out as much of the brown algae as possible every day or two. What I like to do is to use airline tubing and siphon into a clean 1 gallon jug. I would let the jug sit a couple of hours (the algae will settle to the bottom) and then pour the clear water back into the tank. I would repeat this process every night until the brown algae is gone, usually a week or two. You can also add sponges to the tank, and they will bind the silicates into their biomass. HTH, good luck.
