Can I refinish Trex Decking because of a mold problem?

About all you can do is clean it.
Trex has had a real problum with molding, may want to take a look at there web site and see what they have to say about it.
Some people that complaned enough got all new material to rebuild the deck.
http://www.cpsc.gov/cpscpub/prerel/prhtml09/09217.html

If you Google Trex recall there's plenty of other ones.

Trex

Hello, I am a representative of Trex. You can see how to care for your deck in our Care & Cleaning Guide: http://www.trex.com/own/care/index.htm. Just be sure to choose 'Other Trex Products' for your Accents Saddle deck.

Feel free to contact me with any questions.

Brittany
Trex