What is this brown funk?

It's brown algae caused by excess nitrates and too much light for the level of cycling this tank is at. It must be a new tank. Just keep up what you are doing and it will p. My fresh water tanks always do this after about the 2nd month. I usually get a bacteria bloom right before this happens. Something is just slightly off in the balance of things. Keep in mind that too much cleaning is going to remove beneficial bacteria and that's a really bad thing.