What color is it D.B.? I'm pretty sure it's not an algea. But, I guess as long as your fish aren't suffering, I wouldn't get too worried. I once had some brown crap all over the walls of my tank (literally all over) that would come back pretty quick after I wiped it off (more like scraped) but no where near only four hours. I later found out that it was one of my plants emmitting something or other and as soon as I took the plant out (and scraped the ish again) the brown crap never came back. Probably no help, but at least I tried.