What sort of contractor (if any) should I call for this ceiling crack?
There is, or has been a leak - those come from plumbing or from the roof, or occasionally from a water spill on the floor above that was not cleaned up quickly.

If you are quite certain it's not expanding, and you never see or feel moisture near it after the tub is used, or after a rainstorm, you could hire a painter and hope that it was from a one-time spill event. 

If it comes back a plumber (or a roofer) seems like a requirement, and for the plumber to see what's up you will probably need a drywall contractor to patch the holes the plumber will need to make to find out what's going on and fix it. You do not want to pay plumbers rates for drywall repair.

You'll still need a painter, but the drywall contractor might also do painting.