I battled GHA for about 6 months with no luck. I had it worse than anyone's tank I've ever seen...and never kept a picture for proof, thank goodness. Here's what I did...and now my tank is near spotless. I barely ever even clean the glass (and I have 2 flasher wrasses and a blenny). I was desperate...and did just about everything in the book.
Tunze Nano Skimmer -- biggest help
Increased Sump Size from 10g to 20g
Added a Remote Sand Bed -- Second biggest help
Reduced Photoperiod
Switched sump from a 24/7 photoperiod to a reverse photoperiod
Added Chaeto as well as keeping Caulerpa
Reduced feeding (even though I was already target feeding corals and letting the fish just pick at the leftover food)
Phosphate Reactor
Quit dosing everything
Weekly water changes
Took out all artificial sponges/bioballs/etc.
Ran carbon
Took out every single rock & scrubbed the algae off in a separate container of saltwater
Changed my lightbulbs to ensure they were the right color spectrum
I tried a lawnmower blenny, about 75+ blue legged hermits, and a sea hare, but honestly, they can just make the problem worse by spreading the spores of the algae through their poop. Just find the problem...and get rid of it.
Turns out, after all of that...and two months of having a spotless tank...I got a TDS meter to measure the quality of my RO/DI water (compliments of Joel at Aquarium's Etc.). My RO/DI water was OVER 700 TDS. (It should be ZERO.) In fact, the EPA recommends drinking water be 500 TDS or better to give you a reference. So, I changed all of my filters/resins to prevent that whole problem in the future.
Good luck & hope this helped!
Nikki