[Leo.G (Leo)]

Algae is always a hit or miss situation depending on how bad it is. A late, former best friend and neighbour would ask me on occasion to go with him to hold a ladder if he was doing high work (he was late 60s). He ran his own house painting business, all across the blue mountains. Many of the houses had mould/algae/heaven knows what on the external walls and I was happy standing there with the pressure washer or his portable steamer if he wanted the old water logged paint gone too. Not so much fun in the rain or winter.
Good luck with the pressure washer!
Depending on the surface you could apply a brush/spray on sealer which would give the algae less grip and perhaps hose off easier. I've heard this mentioned but haven't had anything to do with it. It's probably just called good quality external paint.