Anyone help me Identify this creature?

I have always noticed a little string coming off of my a piece of coral. I always thought it was slime or something released from a blasto or xenia.

When I moved the rock this past weekend I noticed it was some sort of creature that leads out long strand(s) in the current. It has a trap door that it can close. Here is a pic with its strand out.

Anyone have any ideas?
 
Great thanks for the info...notice how its strand goes right over the birdsnest LOL. Guess I'll have to move the rock!
 
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