Hard to see what's going on there with the fluff. Have you reached in to touch it, pulled a bit out to rub between your fingers to determine its composition? Who lives in the tank, what do you feed them? It could be leftover food that is molding, i.e. overfeeding remnants rotting. My experience has been pelleted food in particular does this. Could be other things, but I would start with removing some and investigating close-up and physical.