Please help ID. Slime?

Good morning!

75 gallon reef, lots of corals, fish include 2 clowns, small yellow tang, and a blue wrasse. Shallow sand bed.

Can someone help me ID this stuff. It has the consistency of mucus. It's photosynthetic, sometimes clear, other times brownish. Main tank is mostly clear except it causes a film layer on top, if the surface isn't agitated. It's clogged up my cheato and kills it. Been dealing with it for about a year. Corals are fine.

Phosphates and nitrates are near zero.

Attached photos: sump where cheato was killed, and main tank the only growth spot on sump return.

Dinos?