Brown algae is another name for diatoms. Most new tanks get diatoms. They are not harmful to fish. Otocinclus catfish eat them. Keep your lighting to less than 8 hours a day, unless you have otos, because if you do they will eat the brown algae as fast as it appears. Of course if you up the light too much even with otos you will get different kinds of algae. If you don't want to get otos, or they are not compatible with the fish in your tank then simply clean the decor and reduce the lighting. As the tank ages they generally go away.