They are nps, true. You have to feed them. Whether you consider them a coral or not??? They can suck themselves in like a nem, they sting like a nem, and they can attach below the sand with a foot or tail of some sort. THEY DO NOT host fish. I believe that is the confusion with these guys.
They have this fleshy feeling tube that you can see in my pic below the tentacles, but then they have the tail that grows under the sand you don't see. That is how they anchor and or move. The outer tentacles are what initially sting the food, then drag them into the inner tentacles that also stig and feed the mouth.
What initially drew me to these guys was the double color. Purple, pink, black with that inner green is beautiful when it is flourest right.