Mold and mildew buildup around the swimming pool is no surprise. There is constant moisture in contact with the deck, and the ambient humidity keeps it from drying before mildew can form. Mildew is produced by molds, which means the best way to deal with mildew is to keep mold from forming. You can rent or purchase a pressure washer to blast off the mildew but it may cause damage to the surface of the deck. Wood decks will splinter and fuzz up. Slate and other porous stones will flake. Brick may become etched. A more gentle method will get the mold and mildew off and preserve the beauty of the deck.
Remove all the furniture, pots and other items from the surface. Wet the deck. Pour 1 cup of oxygen bleach into a 5-gallon bucket. Oxygen bleach is gentler than chlorinated bleach and will not damage any plants that are near the deck. Fill the bucket with water.
Dip the scrub brush into the bleach and water mixture. Use the brush to agitate the solution across the deck. Pay special attention to open grain areas and the places that are shaded. Mold, and consequently mildew, grow best in areas where the sun doesn't shine.
Rinse the deck with clean water. Use a squeegee to push off the excess water so the deck will dry faster. For indoor pools, set up a fan to get the air moving and contribute to evaporation. Allow the deck to dry for a full day or longer until it is completely free of moisture.
Vacuum the deck to remove any dust or small debris. Apply a waterproof sealant appropriate for the decking material. If it is a wood deck get one that also has UV protection. The sealant should also have a fungicide. There are sealants for tile, stone and cement among others. The sealant will help prevent mold spores from getting into the grain of the decking and form mildew.
Dip a paint roller in the sealant and roll it across the deck. Wood decking or any surface with deep grain should have the solution applied in the direction of the grain. Use the edger tool for corners and the border of the deck.
Allow the sealant to dry for 48 hours or the time suggested by the manufacturer. The sealant won't last forever but it is easy to apply and works. You will have to reapply bi-annually or as the product indicates.