Trex is fighting for it's life in court right now: it seems that the material allows moisture in, and begins to mold, mildew and fall apart. This can easily be prevented by using a sealer - I would see Trex for selections of sealer to use - but for many people, sealing/caring for a deck is the very thing they thought they were getting away from when they built with Trex.
I was very dissapointed to learn of this, after I have recommended the product for years!!