Make sure it is in fact GHA and not bryopsis , you can google pictures of both and see the difference .I've been struggling with this exact issue for a couple weeks now. Working on lowering my nitrates via water changes and nitrate reducer (instant ocean) as my phosphates are 1 or 2 per API test kit. I don't believe I'm over lighting my tank but not certain def. has me stumped. I've been gently scrubbing my LR with a soft toothbrush/pulling off little bits of it where can.
How old is your tank ?This is the nemesis I still battle. It is getting better, but still there. I have and am currently doing the following
1) Manual removal
2) Frequent water changes
3) Chemi pure
4) Sea hares - RIP
5) Astrea Snails
6) Kole and Sailfin tangs
7) Lawmower Blenny (RIP - thanks Kole tang)
9) Crying a lot
Some people are new to the hobby or don’t know the abbreviationsReally, I just removed it by picking it all off the rocks to the point that my CUC could handle the rest. I've never seen the stuff again.