I agree, some type of zoanthid. Free coral is good coral.
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Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian
They look like green button polyps to me.I have some that started out on a piece of rock that I got from a LFS a year ago and they were about the same as the pictures that you show of them.They have now turned darker.