I think I see what you're talking about. Does it look kind of fuzzy? Kind of like a bunch of redish brown hairs a few millimeters long? If so, I have the same stuff in a couple spots in my tank. It's been there at least 3-4 months and doesn't seem to be any sort of pest. It gets outcompeted by just about everything, especially my anemone that brushes against it from time to time. It keeps coming back, though. It actually appears to have some sort of skeleton, but that generally isn't very visible unless it gets partially killed off by something.