Hard to tell by the pic but it looks like it oculd be an Oenone worm. Try looking for some pics and see if it is. If so, get rid of it and all others you can catch. They eat snails and other live mollusks by secreting a narcotic like mucus which paralyzes their prey and then devouring them.
Very easy to tell if it's a Oenone or any other kind of Eunicid, it will be segmented and have a big jaw. I didn't see any segmentation in the images, but they are not in focus, so it is hard to tell details.
Sipunculids are unsegmented worms with no jaw, just a bunch of tenticles around their mouth. The tenticles can be completly retracted, and I know with my peanut worms they pull theirs in as soon as they sense any disturbance. How many images of Sipunculids did you look at? That's still my guess.