I've been through this before, after killing large amounts of things one way or another.
You don't want to cut photo period in a display; that just puts more nutrients into the water, which is what hurts corals and caused even darker scrubber growth. Adding more CUC causes the same thing. If anything, increase the period. You want the nuisance algae / periphyton to stay alive and grow in the display, so as to absorb nutrients. And stop all feeding.
Manually removing nuisance algae will work. Waterchange, GFO, Polyfilter too. Even extra bulbs on the scrubber, or a temporary extra scrubber. If the scrubber is black, you can go to 24 hour lighting and clean it as soon as the screen holes get covered up.
Whatever is decaying is big and will probably last a few weeks. Now would be a good time to vacuum / blow off the display, especially behind the rocks. And make sure nothing is rotting in the sump.