^ My mother and late step-father bought a house with a composite ("Trex") deck in the back yard.
It became an annual event cleaning the mold off the deck.
Every year, by early April, it would look like your photos -ugh.
First I sprayed it with straight laundry bleach in a weed sprayer, then used a low pressure "Karcher" electric pressure washer on it.
Cleaned up and looked new when done.
The problem, of course, was that it blew all the paint off the lower part of the painted wood railing that ran around the perimeter of the deck, which required me to repaint the deck railing every year as well. (They were in this house about 15 years.)
I should never have taken the pressure washer to it. It had the effect of "blasting" the top surface of the deck and making it rougher than it was before, providing even more places for the particulate organic matter falling from the sky to call home.
Each year, the black stain problem became worse.
I stopped pressure-washing it. Stopped using bleach. Switched to a product called "Oxy-Clean" (that a guy used to pitch on television), and scrubbed it with a nylon-bristled deck brush, and just hosed it off with a garden hose. Looked great. Did not require repainting the railing. (That was about the last 4 years my mother still lived in the house.)
YMMV
