Take the rock and scrape as much off as possible. Then apply a kalk
paste to what is left. It will burn the coral. Careful not to touch any other corals. Do not do alot at a time as it will cause your PH
This is info I recieved from another xenia victim. I havent tried it yet.
PS. My xenia has made me about $500 since July....including trades, store credit, frag