I am a representative of Trex and wanted to offer some assistance.
I am sorry that you are having a problem with the build up of mold. Periodic cleaning of your deck is important to prevent the build up of pollen/debris that can cause mold. One to two times a year is recommended.
The Corte Clean product is a very good product and works well eliminating mold and mildew. Your deck may need another application of this product. I would recommend another cleaning attempt before attempting to paint the Trex material.