Brown algae can be bacterial matting or a soft or calcagious algae, The bacteria tends to die off suddenly causing ammonia spikes as it decays, it is soft, tends to come off in clumps and is a symptom of poor tank maintainence and over feeding, it shows a high bacterial count in your water which means a lowered resistance to infection in your fish. soft brown algae sometimes shows gas pockets beneath it , it grows slower than green algae but at lower light levels, several types of fish and snails will eat it but it is easily out competed for nutrients by many fast growing aquarium plants, it is better than bacterial matting but again a symptom of poor tank maintainance. Hard brown algae grows slower, is not bad for your fish or water but is unsightly and difficult to get rid of, it is firm and gritty to the touch. Pull out the effected rocks and scrub them thoroughly then boil them, that will kill all three types. Clean your substrate and do a partial water change and clean the sponge or replace the floss in your filter. Feed your fish no more than they will eat and replace your light so that you tank gets 8-12 hours of tank lighting a day and see if that cures it, you may have to repeat this a couple times but afterwards regular tank care will keep it from coming back.