If it is vermetids and they have weaseled their way in between you GSPs, they will likely reproduce to the point that the coral will being to suffer. I had some in between some zoas and they basically took the zoas out before I could do anything. It's not that they sting, but rather extend their webs and irritate the coral, causing the coral to retreat. GSP is pretty hardy, but I would pull the coral out and break all the vermetid tubes off. Look around your tank because if you have them in one spot, they will likely be all over the place.