Try pulling the coral out and give it a soak in fresh water. Perhaps you can get it to walk away looking for better water. If you cant get it off then take some calk and make a thick paste. Smear the paste over the asp till its covered. Note that some calk past will float away. Be ready with a airline hose to suck the little floaters away. You will also need some replacement water. This method works very well whereas joes juice, lemon juice and boiling water only work some of the time.