Lol I don't know what its called or what it is actually doing. However here I'd a few things I have learned:
1.Don't worry about a zoa if its open.
2. Only worry if it is disintegrating, covers in a bacteria/slime/algea, sealed up extremely tight and is tall and narrow or hasn't opened in weeks.
So......with that being said, my guess is it either just caught something to eat in the water column or it is about to expell some waste. Either way I would say it's fine.