Brown "algae" is usually diatoms and not really algae. The diatoms thrive on light, like algae, but need silicates to live. Some diatoms from time to time in the first year or so of a reef is a common issue and not much to be concerned about other than that isn't so nice to look at. But it also could mean that your's won't go away because there is a constant source of silicates being put into the tank. It might be coming from sand substrate, and in that case it most likely ill stop showing up eventually. But it also could be coming in with the water you use for water changes. If you're using tap water, you can test it with a silicate test kit and see if that's it. If it is, then you can switch over to RO/DI water which would be free of silicates. You could also put a phosphate reactor on your sump and use it with Rowaphos which is a good medium that removes silicates as well as phosphates.