My spa cover has mildew starting to grow on it. What can I use to clean it?

My spa cover has mildew starting to grow on it. What can I use to clean it?

 A well-cared for spa cover can last for years, while a neglected one can crack, become waterlogged and fall apart long before its time. To keep a cover clean, stick to a manufacturer-recommended cleaner or an all-purpose protectant: never use household cleansers, steel wool or bleach on the outside of the cover. For mildew stains, use a medium-soft brush and vigorously brush the stained area with a 4 to 1 mixture of water and ammonia, then rinse with cool water. If that doesn’t do the trick, remove the foam cores then mix a bucket of water, a teaspoon of dish soap and one cup of bleach and scrub the inside of the encasement thoroughly, putting it in the sun to dry. Wipe each foam core with a washcloth and put them in the sun to dry.