Xenia can very difficult to remove from the rocks, and any patches of "skin" left behind forms new colonies. That's the reason my xenia and anthelia are on their own private "islands" in the sand at the bottom of the tank. You can try peeling/scraping them off the rocks, but that won't guarantee complete removal. Not sure if aiptasia killer would work, as most are designed to glue the mouth of the anemone shut, and xenias have a lot of mouths! Another technique that people use for aiptasia is to inject it with lemon juice. This might effectively kill the selected colony, but I'm only guessing here...