Trex refuses to acknowledge that they have problems with mold and mildew. My deck is just over one year old and I have had it cleaned three times already with their recommended cleaner. Within 6 weeks the black spots come back and multiply. I had a pressure treated wood deck for 11 years and never had mold or mildew issues ... I just got tired of pressure washing and re-staining every few years. I should've stayed with the wood ... Trex is NOT worth the cost and is MORE maintenance than wood. After seeing several pictures like the one attached, Trex just sent me a copy of warranty explaining how mold and mildew are not covered ... and never offered to send out a representative, and I only live 20 miles from corporate office.
Don't feel like you're alone. I have exactly the same problem, it's probably worse, with my Fiber Composites deck. The response they gave me is the same they gave you. They refuse to acknowledge a problem and will not even give me the courtesy of sending a representative to inspect my deck. I live in NC close to their plant. They basically have ignored me. I just hired a lawyer and we'll see how long they will act like there's no problem.
I also was duped by Trex's false advertising and failure to back up their product. I have black spots seeping out from the boards (installed summer 2010) even on dry areas of the deck with many hours of direct sunlight each day so this is clearly a product defect - not mold (mold likes damp, dark places - not direct sunlight). I'm filing a MA lawsuit for reimbursement so I can replace the decking with a different product - wish me luck. Notice in the attached photo that black spots are only on the Trex decking. None on the white Fiberone railing, or the house siding.