I would highly suggest to move the fan out side of the shower for several reasons.
1. The extra moisture can rust the parts and cause the shaft to rust in place later causing the motor to over heat and die, or if the wires are to small it could cause a fire.
2. Having it on eliminates the reason to seal the shower since you are removing the warm air.
3. It is against code. At least it is in KY.
I would plexy the whole top of the shower and walls so the whole area would be sealed. That way the moisture will all be contained the the shower and you should not have worry about mold as long as your grout is not cracked (you might want to seal it). With the moisture being contained the air will reach it saturation point allowing the rest to collect on the walls and ceiling that will then drain. If the ceiling is slightly pitched it will not collect but run down the wall.
If you have not checked this out take a look at
Cozy Low-Energy Shower