A. Cleaning with soap and water will usually take care of mildew stains, but to kill mildew will require the aid of bleach.
Mix 1 part bleach with 11 parts water and, wearing rubber gloves, apply to affected area with a sponge. Rinse with a damp sponge then dry area with a soft cloth, getting out every last bit of moisture. Opening windows before a bath or shower will help prevent the moisture that causes mildew.