I have used hydrogen peroxide in saltwater for hair algae, bryopsis, and a red turf algae on or around polyps.
I use 30% hp to 70% tank water for about 1-1/2 to 2 minutes.
After the dip, it is very important to hold the frag in front of a powerhead for 10 seconds or so, to make sure there is no trace of the hp on the polyps.
You may have to do the dip a couple times, 3-4 days apart if there is no sign of the algae dieing.