Brown algae is actually diatoms. They are tiny, but cumulative can create the thick layer if uncontrolled. Usually, the available silica from the water limits the numbers, but it's common to see a bloom initially. I wouldn't worry too much about it--clean it manually, and call it good. There are many fish that will eat it, but if it's a temporary condition, you then have to provide food for the fish once it's gone.