There is no such thing as toxic "Black Mold". This is a term designed to sell newspaper, TV, and internet advertising; along with mold testing. Yes, there is mold, black mold, green mold, yellow mold, etc. everywhere on the planet. If you are going to worry about cleaning the deck, what about the fence, and the soil, the plants, etc. If you have a moisture problem inside your home, you might have a problem with mold. Fix the moisture problem, clean up the mold and move on with life. DO NOT WORRY about outdoor mold. You can't do anything worthwhile about it even if you tried.