Looks like they're extending mesenterial filaments--string-like structures full of nematocysts that are used in digestion, aggressive behavior, feeding, and myriad other activities. There are lots and lots of reasons that they do this: some of them potentially positive, a few of them potentially negative, but most of them rather inconsequential. I've seen reactions like this most often after adding a food that they seem to like, or shortly after a water change, but sometimes they do it for reasons that are totally unclear to me. I'm sure there is something they're responding to, but who knows?
I wouldn't worry at all, they look fine to me.
cj