Could be a bobbit worm the coral killer !
http://www.youtube.com/watch?v=Cc971...eature=related
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http://www.youtube.com/watch?v=LMvvg...eature=related
.
I had one that ate everything including LPS corals. was very fast and smart.
Grew to about 24" inch in 3 years and was as thick as my thumb !
it was so powerful that it could rearrange small rocks.
Was very hard to catch, but i got it by removing a large piece of rock.
It AKA as Eunice Worm
http://www.youtube.com/watch?v=Cc971...eature=related
Mine was EXACTLY like that, grew from nothing,(I quarantined my tank for 6 month) and came as a hitch hiker on live rock.
So my lesson/conclusion is NO MORE LIVE ROCK! If i start a new tank it will be ECO rock only !