Luckily for you, I looked into Pagurus commensals a while back, though I admittedly haven't looked for them much. Growth should be Hydractinia sp. WoRMS has some info on the original description here. I downloaded it several months ago (I don't recall from where; just Google and look around if you're interested.) H. symbiolongicarpus is found primarily on Pagurus longicarpus and H. symbiopollicaris is found primarily on Pagurus pollicaris, though they do occasionally live on the opposite host species. The paper also mentioned a third species in New England, H. polyclina, but that species doesn't occur south of the Cape and has a different host (Pagurus acadianus). Positive specific identification is difficult and technical and I haven't tried it.