I just had issues with my tank and my sps pale out. Turns out it was phosphate limited, I didn’t think it possible if feeding to actually have coral pale from phosphate limitation, although it’s common with nitrate depletion but I had detectable no3. Anyway I did something I never thought I ever would in this hobby, hooked up phosphate on a doser and the tank turned around. No other changes as I fed pretty heavy already.
I was still getting algae on glass - it seems the algae or other organisms can under the right situation uptake the phosphate or nitrate faster than corals.
So despite the fact you get film on the glass I would feed more or dose if needed to get detectable phosphate and nitrate.