Ok this is just a case of high nutrients needing stronger light. The middle had actually already grown, but detached because waterfall screens don't hold dark slime very well. Do this:
Clean/brush all the growth away, including the slot and box, and inside the screen holes.
Run the light 24 hours (no timer).
Slow the flow a bit so it does not wash away the slime; all you need is some type of flow across the screen.
Do any other filtering you can to bring nutrients down.
Clean as soon as the screen fills in, which might be every 3 days.
As more nutrients get pulled out of the water, growth will be greener and will start attaching more to the screen. Then you can go an extra day longer for cleaning, and start doing less of the other filtering.