How to Clean Mildew from Concrete
Add the bleach to the water first, then the detergent.
Even though this solution is mild, make certain to wear safety goggles, rubber gloves, and have plenty of ventilation.
Warning: Never mix bleach with ammonia! Combining the two creates a lethal gas.
Apply the solution to the affected areas.
Leave the mixture on long enough for the black or green stains to turn white, but don’t allow it to dry — 15 minutes should be long enough.