Can anyone ID this worm???

[QtJenny19]

I have found two identical worms squirming around my live rock and digesting my substrate. Their tubes are beige in color and very transparent. I can also see some brown spots on the tube. It has the apperance of a feather duster...it uses its tentacles to pick up sand and places it inside its tube. What can of worm is this and is it safe to have in my tank? Any information is greatly appreciated.

http://i6.photobucket.com/albums/y223/QtJenny19/100_0447.jpg
http://i6.photobucket.com/albums/y223/QtJenny19/100_2439-1.jpg
http://i6.photobucket.com/albums/y223/QtJenny19/100_2447.jpg
http://i6.photobucket.com/albums/y223/QtJenny19/100_2462.jpg
http://i6.photobucket.com/albums/y223/QtJenny19/100_2463.jpg

[sir_dudeguy]

to me it looks like a feather duster

[Paintbug]

Synapta sp. AKA Medusa worm.
http://www.advancedaquarist.com/issues/nov2002/invert.htm

[Amphiprion]

It is definitely a Synaptid 'worm' .