I've done this. If you have a local farmers supply, go in and ask for a dairy steriliser called "Chloras", or any chlorine based steriliser. Its normally VERY cheap, I pay about €16 for 5 gallons. (If you're really stuck, bleach will do the same job)
Wear waterproofs and goggles,.. it will bleach clothes and is very sore if you get it in your eyes, even diluted.
Dilute 5:1 and apply to the wall with a backpack sprayer.
Stand back and watch,... no scrubbing at all!!!
Once the green is gone, give the wall a hose down with tap water to remove the residue.
Job done!
If you want to be a bit more eco-friendly, this stuff does the same job,.. but its a tad more expensive:
www.originaltouch.co.uk/acatalog/Rapid_Moss_and_Algae_Killer.html