I too have the spotted moldy trex deck.
We had the deck built in 2007 and it really looks bad. Same thing we clean it and it looks a little better for a couple of week. I read some where that is was believed the mold is coming from inside the decking itself. It sure looks like that to me too. We have full sun on the deck most of time. What is interesting
is that even the areas that are covered by a table or say the grill have the spots. It isn't limited in scope it is every where.