Those are clove polyps. Those are the more common ones from everything i've seen. Not hard to keep IMO. I've got some that are completely green..not as bright of a green, but pretty decent color..much less common according to everyone who's seen them in my tank (they all want them i guess lol).
Anywho, not difficult to keep IMO. I started with about..10, and now have over 200 within about 8 months or so.