You can wash composite, however Trex does not recommend the use of a pressure washer. The use of a pressure washer on the deck surface could damage the decking surface and will void the warranty with respect to any condition caused by the pressure washing.
Pressure washing is not usually recommended for composite decking because getting too close or using too much pressure can be ruinous for your deck. Composite decking material is softer than natural wood and therefore can be “etched” more easily.
We have found using a mixture of soap and bleach does a very good job of loosening the mildew, dirt and algae. After application you can wait 5-10 minutes then with low pressure and a scrub brush wash off the soap.