Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The only tanks I have seen this scum is when the filtration slows down as the filter plugs with debris. In this case the scum is gone within 24 hours of cleaning the filter. I read above where you do have some ripples going on and I have no idea what scum might survive the ripples.
I would remove everything from the tank, one piece at a time (driftwood, rocks, ceramic merpeople, even a couple of handfuls of substrate) and set them up in a bowl of water or a separate tank, if you have one. Even a plastic storage bin. Perhaps whatever is causing the scum is coming from one of the things in the tank, and it is producing so fast that the water movement and filtration cannot remove it fast enough.
Whatever you remove from the tank give it a quick rinse to remove whatever scum got on it as you lifted it through the surface, but do not try to wash it totally clean. You want whatever has been going on to keep on happening so you can see which item might be responsible.