Not as much red in this holiday growth, but there is lots of mashed-up Cladophora, with some yellow slime and some brown slime. This actually should have been harvested/cleaned much sooner, because it just can't grow (filter) anymore because there is no more room inside.
Some tanks have no sump at all, not even on the back. If there is enough floating space on top, and it won't block the lights, a SURF scrubber is a possible solution.