I have that stuff, too. It's brown algae, or diatoms (which are a unicellular algae). It comes off pretty easy, like Brody said. In my case, because I have algae eaters who love the stuff, I just move the gravel away from the glass for a little while and they move right in and eat it with gusto! My shy, sensitive otocinclus catfish will even do it right when I'm vacuuming the gravel, they love it so much. In a few hours, it's gone, presto!