Never did one of those, and I rather stay away from wood ( this is not wood, I know) but sees to me like a regular house wash mix would work just fine.
SH and soap (simple cherry is great) downstream it, let it dwell and rinse, reapply if needed.
But wait for someone more experienced to give you better advice before you do anything to that deck.
Yea, you can X-jet it at a little higher strength than your housewash mix and it’ll take care of it. DON’T use any pressure or you can damage it. Also, make sure you rinse it well since it has metal fasteners that can rust if the bleach hangs around.
Composite decks usually need to be cleaned about twice a year around here. There are more eco-friendly cleaners that claim they work better than bleach but they require hours of scrubbing with a deck brush to get decent results. Just cover it with SH and soap, let it dwell for a few minutes, and rinse well.
Most customers who buy composite decks do so because they were told they were less maintenance than wood decks. Marketing lies, they are. A lot of them are facing lawsuits for those lies.
Go to a pool store and buy pool whitener (shock). It should be 12.5% or even 10% sodium hypochlorite (Bleach). Use 3 gallon of bleach and 2 gallons of water. Soak all the plants with water really good the soak the deck with your mix. Let id dwel for about 10-15. Rinse well!
PS. Not deck brush will be need ot this proess is done right. Let the chemical do the work!
I do a ton of Decks. Dont use 12% or any bleach on this. Go to Sunbritesupply.com and ask for Tracy, (it is a man), and tell him you have a composite deck and you need to order some safewash. The stuff works great.
Spray it on with a pump sprayer, let it dwell for about 5 minutes, (without drying, in other words keep wetting it), then use your black tip from your pressure washer and go straight down each board to wash it off, give it a really good rinse. Charge betwen .65 to .95 cents per linear foot.
Bob is the man. I never talked with Tracy but I have heard good thing about him too. Congrat on the Downsreamimg set-up. You will love it. Just remember, Let the chemical do the work. Im a hyper guy and I can’t stand waiting but I have for that if I wait I get done faster.