What can I do or use to prevent mildew on my wooden patio deck?

I bought a house with a wood deck in the back that has treated wood, that is about 3 to 5 years old but it has never been stained or painted or finished in any way. This past winter (pacific north west) the wood turned green and so slippery no one could walk across it. How can I prevent that happening this winter
The key word here is PREVENT, bleach does not prevent the problem next year -- I want something that is not harmful to the garden, but will make the wood deck useable in the winter months.