Alright, so I've got this brownish stuff on the tops of all my rocks in my tank. If I rub it off it gives off a brownish color to the water around it. After a few days it gets almost plant stuff on it, I think it's just some type of algae. But it also turns my sand a darker color, and gets the sides of my tank brown. It's not hard to get off, just wipe it off with my finger or a towel, but it's every week. Is there something I'm doing wrong? I have seen my mbuna pick at the plant stuff so is it beneficial?