Leon, first you want to know how old the deck is and if it's ever been power washed. Pressure washing removes the factory protective coat from the surface. There are at least two threads on the BB under "Deck Cleaning". Be sure to read through those, lots of good info. there.
As Bob said, there are some great threads on this, but typically, Trex deck is a GREAT candidate for SoftWashing. Tpically the process is apply a 3% mix solution with Green wash, let dwell 15-20 minutes, (don't let dry), agitate where needed, then rinse thoroughly.
Here's a trex deck we cleaned up with the house wash mix we used. Took about 20 minutes to clean the whole thing.
As Bob said, if pressure washed in the past, there could be start and stop marks from the wand. Just let them know ahead of time. That being said, we softwash about 8-12 composite PVC deck a week. And it works great. We use anywhere from a house mix (1%) to a roof mix (3%) depending on how infested it is. Below are some pics of a ramp we did that was pretty bad.
Thanks Chris. Yes we used only a softwash process. The ramp was hit with a 3% mix and let sit for about 15 minutes. The steps with a 2% mix and a 5 minute dwell time. Basically we do the flat surfaces of them first and then the rails and spindles. What winds up happening is the flat get another coat from us doing the rails, while it has been dwelling the whole time. Then we just rinse. We do use a booster pump to rinse, so that helps greatly with deck cleaning.