Turn off your flow and toss that colony. You don’t wanna play with brown jelly. If you have other Euphyllia take them out and give them a dip and use a turkey baster to blow anything off the tissue. Good luck with it. BJ can be tough.
You can try dipping it in lugol. Once they get it, they are usually gone. Try and the infected head off with out shacks no the stuff off onto other pieces of the coral. I would heat a small piece of glove and stretch it over the piece and tie it with a band before removing it from the tank. Then cutting it off, and dipping he colony.