How do you remove mildew and dirt from Trex decking?

Best thing to do is call TREX and ask them

Spray deck with soap, then follow by gently scrubbing each deck board with a soft bristle brush. Spray/rinse each individual deck board using a fan tip no closer than 8" from the decking surface. RINSE THOROUGHLY. If dirty water from cleaning is left to dry, this will cause a film to remain on the decking surface.

Pressure wash with Krud Kutter from Lowe's

Composite deck. I would power wash it. Use a mixture of vinegar, liquid soap, and water. A good deck cleaner would work. Nothing with bleach. It can affect the color.