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.
This SURF2 or 2x or 2xx is shown after being pulled out of a cylinder tank during a water change. Since there is no sump, the SURF floats at the top. And since this is saltwater, a SURF (which has strings) is ok; but if it were freshwater then a model without strings would be best.
This looks to be a SURF2 or 2x with some Green Grabber® screen cut up to fit in with the strings. Some have said this helped, and others said the strings were enough by themselves.