regular painted wood.
Got that gray looking mold/mildew on it.
It's not filthy, just 1 coat of house wash mix would take care of it.
Decks a pretty decent size, bigger than the patio.
Probably 20x15 if I were to guess
I would soak the deck and keep it soaked, flooding it with water as your mix comes down. Downstream a light mix a couple times shouldn't be a problem. Tarping is another option. As well as including a deck clean.