Currently wrestling with Trex "specialists" about white spotting on my mother's Trex benches and railings. She replaced her 1980 wooden ones in October 2007 with Trex Accent in Winchester Gray. Looked great, weathered nicely until this year, when virtually every horizontal and vertical surface developed white spots. Unlike other posters here there's no sign of black mold. I washed off a year's worth of pollen and dirt with the officially recommended mild dish detergent, medium-soft bristle brush, and water from the garden hose with one of those extra pressure nozzles on it. Hardly a power wash by any stretch of the imagination.
After waiting through 3 Customer Service calls I was referred to a so-called Specialist, who at least gave me an e-mail address to send the photos to (I had taken some with my cell phone). Despite his assurances that I would hear from someone promptly it's been 24 hrs. and no response, not even a courtesy email to let me know who had received mine and was working on it.
The white spots disappear when the Trex is wet but reappear when it dries. You can't feel them as being any different from the rest of the surface with your finger or fingernail. They don't seem to be ON the Trex so much as part of it. Still waiting to find out if we can power wash this Trex Accent or not; online advice is conflicting and none of the techs I spoke with yesterday could figure it out without "more research".
Meanwhile, the same year my mother had her Trex Accent built, we had our deck railing tops replaced with Evergrain. None of the contractors I interviewed for the job would touch Trex, and the Evergrain is still performing as advertised. A light wash once or twice a year (by hand & brush or garden hose & brush) cleans it up nicely, the color fade was no more or less than advertised, and there's been none of the cupping and warping in the hot CA sun that we had with the wooden rail tops. Absolutely NO mold, moss, or anything of that nature even underneath. Both our Evergrain and my mother's Trex are under oak trees, so if the white spotting was caused by tree resin it should have turned up by now on our Evergrain in CA.
Photo shows a portion of my mother's bench.