Oh boy. The GFO will definitely help. At this point manual removal is definitely a must. That definitely looks like bryopsis which is hard to kick, as well as caulepra. That said if you can strip the tank of nutrients, you should see a huge improvement. If it was me, I'd do a tank cleanup. Baste rocks, siphon sand and sump, manual removal, 50% water change, and up the waterchanges and GFO.
That said, don't put a mess of GFO in there. Put the recommended amount and change more frequently rather than a ton of it all at once.