good evening santa monica I am reading a post where you commented on this situation.
(Green growth on the glass does mean low nutrients near
the glass. But brown sps means high nutrients near the corals. If you have a lot of rock, which does act like a phosphate sponge in and out, what could be happening is phosphate flowing into the rock during feeding, and then flowing out of the rock and hitting the corals. It's a stretch, but would explain your situation because the phosphate has no place left to go.)
I have exactly this problem my phosphates are undetectable with several tests hanna phosphorus and salifert phosphate, and my sps are a little brown with retracted polipo and crystals with green powder, how can you solve this problem that I mention? I am currently using a surf2x without algae growth, thanks