Save the time and effort of using a pressure washer and get yourself some "Dairy Hypochlorite".
I used to use a pressure washer, but since I've discovered this, it's no longer used.
It's basically a strong bleach, but it's quite reasonably priced.
Dilute it accordingly, pour on to the patio and work it in with a stiff brush.
Give it 15 - 30 minutes to do its stuff, then rinse it off with water.
I used to have some green marks on my steps, that my power washer couldn't remove. When I used this stuff, the green bits literally just fell off :)