ID Please

[arizonaocean]

Not sure what it is. Just started getting big one day and hasn't stopped growing yet. It's about 1/2 inch long and is growing rapidly!

ID? Is it harmful to my reef? My Corals?

Thanks!

http://hollidaybrothers.com/DSCF0002-2.JPG

[arizonaocean]

http://hollidaybrothers.com/hollidaz/DSCF0002.jpg

[graveyardworm]

Looks like an aiptasia, alone it is not harmful, but they can multiply rapidly, and in large numbers they can be harmful. Peppermint shrimp have been known to eat them, and the berghia nudibranch (http://reefkeeping.com/issues/2004-01/ac/feature/index.php)