brown color in a new tank is common, and more important than the color is the texture. is the brown coating sort of fuzzy and chunky, or is it more like a smooth coating of the sand, almost like the sand itself is changing color? the chunky stuff is algae feeding on the ammonia and random wastes, but a smooth coating is diatoms... totally different, not even similar. diatom blooms in tanks ARE due to the dissolved silica in tap water, but they are harmless, and are actually a good food source for filter-feeders. the "brownish" algae is a normal byproduct of fluctuating nitrogen levels, and will go away after the nitrogen is cycled and removed, but if you dont like it you can stir it into your sand and blow it off the rocks, but you cant really remove it or kill it... best bet is to be patient and let nature take its course.