My pressure-treated wooden fence looked exactly like this. A pressure wash without any soap or chemicals, made the brown/gray stuff come off and restored it to the original wood color. Around 2000 psi with a 0 degree nozzle should be fine.

Best of luck!

It still looks nice 6 years later.

I used a cleaner/brightner.
Let it dry.
And now painting an oil based transparent strain.

And all of that lovely soft grain in the wood remains intact.

Superdeck=excellent choice!