They're pretty hardy little buggers and I don't know that it's a sign of anything.  It wouldn't hurt to run some tests and check on levels just to be sure.  That's always my first action.  One thing I can say about the GSP I have, - when it' disturbed it can take a surprisingly long time to open back up.  Not sure why but the colony I have will close up when a crab crawls across it and will stay closed up for quite a while.  I've even had the same thoughts you're having but it turned out to be nothing.