The stringy polyps are called mesenterial filaments and there can be a variety of reasons why corals extend them. Most of the time it's stress related, but like allsps40 said, it can happen when feeding heavily as well or even when corals are battling each other. Sometimes my corals will do it if I'm messing around in the tank for too long. If the coral is otherwise healthy and the system stable then I wouldnt worry too much about it. It's time to get concerned when the coral does it for days on end and it leads to recession or burnt tips