looks like Typical white wood mold. 20% bleach or 30% borate mix will get it killed then make sure the wood is dry, then treat the deck with 2/3linseed oil and 1/3 paint thinner or turpentine) mix. Spray the deck with the Hudson sprayer until it gets a little shiney, but make sure it fully absorbs within 5 secs +/-. Otherwise you will have build up and that looks crappy. If you get too much on just use a roller to remove the excess. This is the standard treatment for this and has been since I was a kid (before electricity)
There are a lot of types of wood mold it could be but, that is hard to tell by looking at it. This type is rarely dangerous to your health, like some of the black varieties can be.