I installed a composite deck off the back of my house two years ago. I will not mention the brand. After the first year i noticed black mildew spots. I tried everything available (that I could find) but nothing removed the spots. The only option that i had was to pressure wash the entire deck with the nozzle literally one inch from the surface.
This removed the black mildew spots but I was left with a dried out looking composite deck that was in desperate need of some kind of protection.
I found a composite stain at home depot, it was a behr product, I gave it two coats and to my surprise it came out stellar. The deck boards looked brand new.
One year later its still holding up fine.
So there are options for your deck. Wether its mildew,sun faded, or just weathered.