Anyone know what this worm-like thing is?

[alex277]

I recently got some new fish and since then have seen a couple of these guys on my rock work. They seem to stick to the rocks and stretch in and out? They have these tentacle like things at the head. I really have no idea what these are. When I come up to the tank they retract and hide. They are fairly small, less than an inch in length and super slim

 

[AScientificReefer]

Seems like a medusa worm which is actually a type of sea cucumber, they do have toxins they secrete if they die or feel threatened but they also filter feed

 

[JoJosReef]

Hard to say without a closeup of the head. Do they have 4 or 5 tentacles? Are they pointing straight out or curved? How would you say is its retraction speed? Lightning fast at the first sign of movement?