Your water parameters look fine. I had a similar issue in a 35 gal. almost 4 years ago.A friend of mine gave me some CarribSea base rock that he used in his fish only tank and kept dry after parting off the tank. Within a few weeks all my LR, glass, and SPS corals covered by hair algea and cyano. It was nasty. After making a big amount of water change and trimming i put 250 ml Seachem Phos Guard in a filter bag (more than reccomended amount) and started to run it in an AC 500 HOB filter. I also used Red Slime Remover for cyano. I don't know which one worked for me but within a week or so both were gone and never had any problem after that. Maybe none of them worked it was just a miracle
It sucks but when GHA is introduced to the tank it is realy hard to get rid of it since it can utilize the minerals in the salt and proper light etc. and continue to grow even if everything is fine.