If that's all you got you can just scrape it off or treat it with H2O2, just be careful you don't squirt it on the coral.
Yes treat it out the water please.
I had the same on my Duncan. I was going to remove it from the rock and dip it in H2O2, but the frag was glued solid to the rock. I scraped it with a toothbrush and kept at it every time it started to come back until it just died off.