Hi Dee,
There is a great outdoor cleaning product called 30 Seconds. It kills Algae, moss, mold, mildew. I have used it myself. Just pour some of the 30 seconds into a pump spray bottle and spay it all over your timber deck and leave it for a couple of minutes, Then spay it off with your water pressure washer.
Have a look at the site below for more details
http://www.30seconds.net/