I have used emerald crabs in the past for bubble algae but never GHA. Sea Hares are in fact awesome, careful it does not starve they will devastate algae faster then you would think. I would really attemp to treat the problem creating the GHA out break. Could those few rocks be leaching? Are they maybe a different rock from the others? I only ask because I have had this problem from a few lps covered rocks I rescued that ended up giving me problems.
The real green hair algae king is the sea hare. And you only need one to do the job. I had to give mine away after destroying my hair algae problem lol.