Those are a form of button polyp. Much prettier than the average brown w/ green center, but just as invasive.
Same as the camo paly, but instead of beeing an ugly green/light brown (much uglier than the average button polyp) they are light blue, and can be light greenish under lower lighting, or when they are shadded by a large colony of them. I have always been a fan, but not in a small tank, where they can become problematic.
They are commonly called "texas trash" where I am from.
Agreed on the post about spreading throughout the tank. These are one of the types of palys that tend to detatch from rocks, in small groups/individual polyps, and re-atatch in other locations of the tank, and can possibly become a pain in the but, as far as taking over other corals/polyps/realestate.