Knee jerk reaction to many hitchhikers is to remove. Without seeing a picture and hearing slug like on coral, I'd guess flat worm or nudibranch and advise that you remove it too, so I wouldn't give the LFS a strike for that.
This one is a good hitch hiker though. I've got a bunch of them in my tank, not a clue where they came from. You'll rarely see them, they come out mostly in the dark.