GSPs are pretty much indestructable. I have had them for years.
Closing for a few days is normal. If they don't re-open in a few days I
would suggest doing a partial water change. It can't hurt and probably
will help. For my reef, I know it is time to react when several
different corals don't look right. With time you get to know your
creature's habits. When corals or fish are under stress they will let
you know. For me, 9 times out of 10, a water change gets things back to
normal.

I would not worry about them. They are very hard to kill. I recently
took 1/3 of my reef into the LFS to exchange the rock and corals for new
clean rock. The star polyps were taking over my tank. Once they
establish themselves and start growing watch out. A lot of people
quarantine their GSP rocks by themselves in a reef so that they don't
spread too far. I now only have it growing across the back of my tank
glass. It is not touching any rock. It looks cool blowing in the current.

Good Luck

Todd