Yea, I agree with Toxiq Reef. From those new pictures, it definately looks like a medusa worm. They are said to be reef safe as they scavenge for waste but as I understand it, they are in the sea cucumber family and are known to have toxins in them which they can release if they are stressed or die. Those toxins can take out your tank or kill fish from what I have read in the past.
I'll defer to other peoples experience with those creatures as I have no first hand experience with them.