my 180 used to be completely covered in Xenia with the exception of a full 6" of clearance around a Hydnophora rigida, NOTHING would get close, they have one heck of a sting when sweepers are extended.
as for trying to isolate it on the little island in the sand, it doesn't work, even a fish picking at it will break a small piece off and it'll start growing here and there, I've had it make it to the fuge and start growing. if you don't want it everywhere, don't add it to the system.
kc