According to the U.S. General Services Administration, you can clean mildew from stucco surfaces with a mixture of 3 ounces of TSP, 1 ounce of Tide and 1 quart of laundry bleach mixed with 3 quarts of warm water. Apply the cleaner to the affected surface and use a scrub brush to remove the stains. Rinse the solution off with clear water.
A mixture of 1 part ammonia to 3 parts water may also be used to clean mildew from stucco surfaces. Make sure to wear gloves and keep a bucket of wash solution nearby to rinse the skin in case of accidental contact. Keep cleaning solutions away from dogs and small children.