Brown algae is diatoms. They don't last as they are caused by raised levels of ammonia/ammonia spikes. If they last, you have a problem. If you leave it uncleaned, the diatom phase will die out. The left over film will eventually get colonised by proper green algae, and the lot will turn green. I have a hillstream tank so I am trying to grow algae on purpose in there. It doesn't look good and I can barely see the fish as I don't bother cleaning the front glass either in order to provide more surface area. But the fish are happy and healthy.