Hardly impossible to kill. If you want to get rid of it, use the same stuff you would for aiptasia. It'll kill it back, may take a couple applications. Best thing, as others stated, is to keep it off the main rocks, because otherwise it is going to spread. Even when placed as an Island, I've had it jump an inch or more to spread, so make sure its plenty far away.
Also GSP grows with a mat that can be pryed and pulled off of rocks (very difficult to get it all however. Easy to frag, simply cut and remove part of the mat and superglue it down to whatever rock you want it to grow onto. HTH