Hard to stop well established GSP...it can be a wild weed and hard to kill. Mine never came off easy like the others experiences, unless it is trying to jump to another rock and then you can catch it exposed. Once if covers a rock it is on like glue.
If not, surrender the turf. Take a small head screwdriver or the like and try to get some rock with it and chip it off. Glue to another rock. Kill GSP, or isolate to a rock that does not touch another one and keep it trimmed.
If it is just a small amount, you can put some kaulk paste on it and it will kill it.