Stay away from Trex Decking. I've seen countless messages and blogs on how bad Trek is when it comes to mold. Unfortunately it was too late for me. However, it seems that it depends on the batch. When the deck was first put up, last year, it wasn't long before the black spots attacked. Having a dispute with the builder, it would have been useless to involve him. Two boards where put in toe to toe with no gap. To get them off I ripped into the holes pretty good so I replaced the two. Here we are two years laater and I'm still having mold problems with the decking, but not the two I replaced.
I contacted Trek and they do not have a sufficient answer. Per TRex,To simply say that Trex decking material does not inherently hold a food supply for mold and that the food supply must come from somewhere else is pointless. Why is it that other "green" decking does not have nearly this issue. I'll never purchase Trex again and have already convinced 5 others not to use Trex. Actually they just saw my deck and did not need much convincing.
I clean my deck with the suggested cleaners at the appropriate intervals. Doesn't work. Only a bleadch type product works and even then, I only get a few months before the black returns.
Just do yourself a favor and use something other than Trex.