I'm afraid I don't know the exact species, but it's a Ceriantharian - a tube-dwelling anemone (not a true anemone). The white part displayed above the tube in the first photo you posted is the inner "whorl" of tentacles (you can still see them down around the mouth of the nem in your follow-up photo). I suspect you saw them up like they were because it was eating, but I could be wrong. Your specimen has quite a few distinctive features (low number of transparent outer tentacles, white inner tentacles, the shape of the outer tentacles, and the patterning on the tube are the ones I can see), so if you could reach out to someone who knows a lot about these guys, you might be able to get an ID - especially if you know roughly where it came from (i.e. if you know what ocean it came from/what area of what ocean).
If I had to guess, I would guess it's a member of the Arachnactidae family, possibly Arachnanthus or Isarachnanthus genus, but that is just a guess.
If it hasn't moved at all since you first noticed it, I would assume it's a filter feeder type of tube-dwelling nem, so it's most likely harmless.
