Maybe, is it super tough to remove? Like turf algae?
It is hard to see what it is in the photo.
I have a new tank and it is in the maximal uglies right now (all dry rock start and just a few months in so I was asking for it!).
No LFS here to buy CUC from... derp.. closest is a 2 hour drive... shipping is rough on CUC too. Ya, I didn't think this out too well starting a new tank before winter, did I? lol
Finally added a pincushion urchin last night and it has already mowed a spot bare.
If nothing will touch yours, I would take a closer look to see if it is a really tough turf or bryopsis. Some algaes need more aggressive removal than a CUC can offer. Still surprised your rabbitfish is so lazy...
If it is bryopsis, than reef flux may be warranted, it is very difficult to eradicate but reef flux specifically does a good job on it. It doesn't work so well for GHA though. I would trust it more than algaefix if I had to choose.
The fact that is it growing like little clumps or bushes wants me to say it is some sort of turf algae and not "normal" GHA.