It's an anemone but I forgot what kind it is. I believe it's some kind of ball anemone. I have 2 in my nano tank they only get about an inch including the tentacles. They have a powerful sting from what I've been told. I've had mine in the tank for about 8 months and within the past couple weeks have noticed two more in the sand bed so they DO spread. I don't think they are in the aiptasia family and when I got the two extras out they didn't spread more.

Personally I think that they are really pretty and they have not disturbed the fanworm near them. Your choice as to whether you get rid of them. Mine have done no harm so far.

Here's a blurry pic of one of mine.