Old thread or not, the manufacturer’s recomendations usually ends the discussion pretty quickly:
“How to remove Mold from Trex® Composite & PVC Decking
All exterior building materials require cleaning. Trex is no exception. Periodic cleaning of Trex decking will remove dirt and pollen that can feed mold. If mold colonies appear, clean the deck with a commercial deck wash containing a detergent and sodium hypochlorite, commonly known as bleach.”
https://images.trex.com/is/content/trexcompany/trex-mold-technical-bulletinpdfpdf
