That light-colored patch is cleaned concrete. I purposely did that section in the middle of a patch of un-cleaned concrete specifically to highlight the contrast.
Now the entire patio looks like that.
It took me hours to clean the patio today, not the least because I had to rake and sweep first; but when I got to the part by the back door, the raised part, that required that I go inch-by-inch to get it clean.
I know why that happened, though: a few years ago the maple trees all caught some kind of fungus and were dribbling sap everywhere. I discussed it in a blog post, where I had to basically soak the Escort with the hose and scrub to get it off the car; anyway sap landed on the back patio, too--and because it was sticky it accumulated dirt. And the combined dirt and sugar was a nice place for moss to grow, which is why it looks slightly greenish.
It's still kind of surprising to look out through the back windows, though, and see a nice white patio.
Learned two things, though. 1) if I keep needing a pressure washer, I'm going to have to upgrade to one with more oomph. 2) always assume it will take at least twice as long to pressure-wash something as you think it will.
Oh well. Time for bed.