GHA problem suggest high phosphate and high nitrates. Nitrates you can lower with a series of water changes. Phosphate can be pulled off with phosgard or GFO reactor. If you want to get really technical, look up 16:1 Redfield Ratio to get the right balance with refugium or algae reactor.
If I do a series of water changes over a 1-2 week period, blowing off rocks too, I tend to capful dose Dr Tims All in One to bump my bacteria load. Good luck!