IME they will not do much on a "large" bloom of HA. IF you address the underlying nutrient source (could be phosphates in the rock, could be overfeeding, etc.), and manually pull the algae, they will do a pretty good job on getting the newer growth down. I kept about 6-10 in my 125 at any given time. Only had 3 out of maybe 25 ever go bad and start eating zoas, but it is like watching a turtle eat--you can pull them off pretty easily and move them to the sump.
It is interesting reefing. At some points, we fear the tiniest bit of snacking from fish and critters that might taste corals, and at other times we are breaking off huge pieces and giving them away to make room.
I would try 3 or 4 in your tank along with the other stuff.