Have you tested your P04/Phosphates, and with what?
My nitrates register 0, my P04 was on the high side 0.11 after already running GFO in a reactor, and doing multiple water changes with 0 TDS RO/DI. I can only imagine that they were much higher when the algae began.
Been in this hobby for quite some time, and for the past 6 years I had no issues with GHA.
My issue is due to a stupid mistake that killed all my fish, and when I was unable to remove all of them that drove my reef out of whack...
I am working on correcting it at the moment, and not with fish or any invert that will die once all the algae is gone.....
Check your ro/di TDS, and your P04........