Mildew grows in grout because it is a porous surface that can be constantly wet. Mildew is also often found on bathroom ceilings because of poor ventilation and a ready supply of food and moisture. In addition, mildew can grow behind caulk if its watertight seal has been broken. Unfortunately once mildew moves behind the caulk, it is hard to penetrate with a cleaner. If this is the case, you should get professional cleaning assistance.