המועדפים של bartwursten

Is it a fish? Is it a slug? Is it a fishy anemone? I do not have a clue what this is!

It is about 30mm long and there were a few of them in the sand - outgoing tide nearly on the turn. Most were buried and only the "fan fin" was showing.

Resting on the wet sand, when the sand collapsed it arched it's face upwards (2nd and third photos) and seem to spawn capsule from somewhere - there are 2 floating in the 4th pic.

Totally hypnotic, by the time my sister-in-law and I carried on the brother had walked 2kms away from us!