A deck goes grey for two reasons at once: the surface of the timber weathers, and algae settles into it. Cleaning deals with the second and reveals how much of the first there really was.
Composite boards take a gentler approach than people expect. They scar, and the scars do not weather away. Wood can take a little more, but only along the grain and only up to the point where the surface fibers start to lift.
Get it wrong on wood and you get a furry deck that splinters underfoot for a season. That is the single most common piece of damage on a badly washed deck, and it is entirely avoidable.
Porch ceilings, stair treads, the underside of rails and outdoor shower floors all collect more than the open boards do, because they stay damp longest. They are also the parts most often missed, and a deck shows it where somebody stopped short.
Stain and sealer need a clean, dry surface or they will not key properly. If the plan is to seal the deck, have it washed first and give it a few dry days before the sealer goes on.
Prices are on screen in the estimator, and nothing is due until the work is finished. Book the date yourself when you are ready.