Best type of insulation for bathroom?

Its a mobile home. Bathroom is 5' wide, only one exterior wall. There was previously mold in this area, so we had to replace the exterior siding. What type of insulation would be best here? I am afraid fiberglass (which we are using everywhere else in the house) would create black mold if some steam were to seep in, and with this right behind the shower, it could happen. I will be putting in the new insulation, and then mold-resistant 3/8" drywall, then paint, then an acrylic tub surround with a refinished vinyl tub. At the top of the surround, there is a window that is used instead of an exhaust fan. I dont like it, but there isnt much i can do about it except tear out the ceiling and i dont want to do that.

What type of insulation would work best?