A professional what? We're assuming it's a roof thing, but what if it's a plumbing thing? I assume you need someone to cut apart the ceiling and look around to find the source of the water. I have a similar sitch, except I have a master bathroom above my moldy spot (which is in the garage), and I think it was old caulk that I recently replaced. But I'm not sure who to call to take apart the ceiling, remove the insulation and start poking around to determine if I fixed it with new caulk, or if it's an ongoing issue.