I had a bad gha outbreak after my tank maintenance guy forgot to plug my heater back in for two weeks. I took out my rocks and scrubbed them with a stiff brush in a bucket of tank water. I siphoned all my sand. This was hard to do with some rocks that had frags on them. I had to do a few rocks weekly for a couple weeks. This made a huge difference. I also got a tuxedo and long spine urchin. My tank was covered, not as bad as yours. Gha, red and green turf algae. The urchins are awesome. Here is a pic with coralline on the glass, no judging, lol. Tanks acrylic and 19 years old.
Only a couple tufts of gha left.