Mold Popping Up 1 Year After Rehab

[Jason Measures]

Hoping someone can identify this issue for me. I extensively rehabbed a property beginning of last year and my tenants moved in mid-March. The bathroom was completely redone and a new exhaust fan was installed, but either mildew or mold began popping up in the ceiling of the bathroom recently and it appears to be spreading. The exhaust fan is not tied into the light, so my first thought was that they never use the exhaust fan and the moisture is building up and creating mildew/mold in the ceiling whereas the exhaust fan would properly dispose of if used properly. Based on the photos, can anyone identify the root of this issue and/or provide a solution? Thanks for all of your help BP.

(Sorry for the 2nd photo being rotated improperly)