Best of luck. Getting good advise.
Aside from the algae, your rocks and sand appear very young, given that it's a nine month tank?? No coraline yet?
Agree with others, running your whites that high probably is not helping.
You have a pretty robust CUC, but if they're fat and happy on mysis, probably not eating too much algae.
Believe it or not, running 0 (or near zero) on phosphate can encourage GHA in a fuged system. The chaeto can't compete with the micro algae when you have very low phosphate. Given that your rocks are so accessible, I wouldn't be able to resist just pulling them into a little bucket of saltwater and having a serious go at them with a wire brush and a sprayer of hydrogen peroxide.
I've heard that turkey bastering your rocks every week or so helps to dislodge and crud and gunk algae may feast on. Good luck, you'll beat it!