Same here. I know this is an old thread. My sump has black walls. Doesn't get any light except some red led from my algae scrubbers. My filter media is saturated with the stuff. It is below the skimmer intake, and doesn't appear in the DT. I do get a rust color on the glas that doesn't come off easily, by I can get it off without damaging coraline. I'd say there is easily one to two inches of the stuff at the bottom of the sump. It's like ghostbusters pink slime.