That is not GHA. GHA is very fine, thin, stringy.
I just learned my lesson about that Algae.
I may be wrong......but here is what happened to me.....and what I recommend based on my experience.
It's Lettuce algae. HORRIBLE STUFF!!
It's thin, sheet like. Like Lettuce leaves.
Whatever you do, is scrub it off. Not a smear left.
Better yet, cut off the Frag and throw out the Plug.
If you have any growing in your tank. Get it out.
I just struggled it with it. Same like you, I had a bit.
It spread for me across my rocks. And it smothered my short corals.
This is when I had the outbreak. Few weeks ago.
See all the Green Areas.
I eventually went it and removed all my Frag Plugs on my rocks.
I Used a very rough nylon brush and scrubbed my rocks clean.
After the scrub, I added a couple of Turbo Snail, and and Emerald Crab, to ensure the eat any I didn't get.
(Lettuce wasn't the only reason for removing the Frags....It's a long story you don't want to hear..but if you do...)
https://www.reef2reef.com/threads/a...er-option-move-corals-to-holding-tank.736439/
Hoping that stuff doesn't come back.
I actually spoke yesterday with the Frag Seller who sold me my first Frag with the Lettuce Algae on it.
He also confirmed, he got an outbreak, and rather throw out a Coral on a plug then get Lettuce Algae into his system.
MAYBE I'm wrong.
People buy the stuff.
https://www.liveaquaria.com/product/2598/?pcatid=2598
Folks grow it in the refuge, kind of like Cheato.
https://www.reef2reef.com/threads/lettuce-algae.572330/
Your decision. Other Folks may disagree with me.
