I went through a long battle with GHA myself. And I've heard all the reasons people say I suffered. The first and most common reason was excess nutrients. Well, I had 0 nitrates and 0 phosphates (Hanna Checker) throughout my battle. Its all being consumed by the algae they said, but I was feeding about 1-2 times a week (I lost over half my fish and all my shrimp due to starvation), so that wasn't it either. Too much light was the next excuse, so I moved all my corals to a separate tank with the light, shut all the lights off in my DT and wrapped it in a blanket to keep out ambient light. Guess what, algae did just fine. Next I put the corals back in the DT and removed most of my rocks (the ones with the worst algae) and put those into a Brute can with a heater, circ pump and an airstone (to keep water oxygenated) then covered it with the lid. after 4 months in total darkness, only opening up the lid to do weekly water changes and a trial H2O2 daily dosing ritual, the algae was as green as ever. Reducing the lighting will not kill algae, but it may help prevent new growth. BTW, hydrogen peroxide does work on a VERY target specific scale, and is NOT worth the effort in a DT.
So after a year of many tests, trials and tribulations, I discovered there is only 1 efficient way to remove algae. Manually. Fortunately a very robust CUC consisting of specific types of hermits and snails will do all the manual labour (converting all the green grass proteins into poopy ammonia). You simply need to remove the ammonia. And I found an algae turf scrubber (ATS) to be better even than a skimmer. My skimmer and phosguard reactor have been sidelined into storage as the ATS is sufficient.
So, keep you feeding schedule as is (assuming 0 times per week for a coral only tank),
keep the whites reduced for now (to prevent new algae growth)
get a healthy amount of hermits (Dwarf Blue legged, Electric orange/Yellow/Blue, Scarlet), Turbo snails etc. Maybe even an urchin and/or Lettuce Sea Slug.
Good Luck. I won my battle after I tried everyone else's suggestions although the tank did suffer in the meantime. Watch your Alk
/Calc levels too, so the corals don't suffer extra.