Try not to remove it while its in the tank, any stray particles can become colonies. If you can take the "infected" rocks out and scrub them in a bucket, this method has been effective for me.
Just scrub the colonies off the rock?
If you want to be sure and quick, a hand held butane torch spot sanitizes live rock really well. Harbor Fright has them cheap. Allot of times even a toothbrush can't get every piece out of every nook and cranny and it can come back.
I could trade to a few nice pieces of live rock for the ones you have with the xenia on it.