Having mold on the tile in your shower is different from having mold on the drywall around the shower. Mold on drywall would be a much harder thing to get rid of and may go down deeper into the wall than you can see.Someone (<b>not you!!!</b>) should cut out the affected area of the drywall and see how bad the mold problem is. If it is limited to just the drywall then they could patch the drywall back up. If it is on the studs then you have a bigger problem to deal with.