Without seeing it in person, my initial reaction is majano anemone.
Majanos are considered a pest anemone and can quickly multiply and overtake a tank.
Joe's Juice, straight lemon juice, and kalkwasser paste have all been reported to kill these.
The ones in your picture are very pretty. There is an LFS in town where there are a couple just like those in a show tank that the LFS owner won't kill because they are so pretty.
Fortunately the owner won't sell them to anyone either so they won't spread to another reefer's tank.
Google majano anemone and you'll get several articles about these. One article I found even said that tanks with good water quality, low nutrients, and proper maintenance could in fact keep a few of these without any problems. I'm not sure I would want to take the risk.
