Looks to me like the deck is fiberglass composite or Trex material, how would you go about cleaning this?
There’s a couple recent threads on it. 
2-3%. SH and surfactant. Dwell for 15 minutes keeping it wet here and there.
1,000 PSI to rinse/wash.
Newer trex I usually get away with just rinsing but your pic appears to be 1st gen composite decking and doesn’t clean up as nice.
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You can and probably will lighten the color of the deck. Just let the customer know ahead of time it’s possible. The dark circles aren’t going to go completely away if they aren’t organic. They could be tanins from wood fibers. I’d hit it with 50/50 sh and water. I also tend brush a lot of it and clean with light to no pressure. It will look stained while wet. Let it dry before you get frustrated. 1st gen composite sucks. Let the customer know what to expect and dry to over deliver.
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