In Prince Kaida's tank on all the oldest decorations and in the filter, there is this brown build-up stuff. I've thoroughly cleaned everything that had this stuff on it, but it always 'grows' back within a week. It's not really unsightly and there's not too much of it, I'm just curious. The same brown stuff is in the old filter cart whenever I change them, and it has also started inhabiting the filter media sponge I have baffling the filter output. Nothing bad has happened since it started appearing. Is this good bacteria just colonizing everywhere it can or should I be worried?