Does it have really fine clear threadlike things coming out of the tube? You might have to look at it at night, or at a different angle to see them. If so, then it's a type of hydroid. If not, then I'd go with the tunicate.
Regarding the aiptasia, peppermint shrimp *may* eat little ones, depending on its mood. But if you've got an infestation, I'd go with the kalk paste or Joe's Juice pronto! Then once you get all the ones you can see, the peppermint shrimp will get all the ones you can't see!