Hi Blacksheep, I had a problem with algae also, With a patio not a wall and I tried different options, I would try the bleach option first , The thicker the bleach the better, If that does not work then the jeyes fluid might, Sophie and Greengrass are offering the correct advice, But if all else fails The only thing that worked for me was a product called moss-go, You can get one for moss or algae,The bigger stores like md o, sheas, woodies or b and q should stock this, It is not the cheapest one on the market but it is well worth the 40.00 euro I paid for it, You dilute it 1 part moss go (Moss go algae in your case) and 4 parts water and apply with your average watering can, You might need a ladder to do this in your case, You can only apply if we are due dry weather for 24 hours as it needs to set in and then let nature take its course By that I mean the rain will wash it off for you, If it is very bad use a bucket and a hard brush to apply, Again the rain will do the rest, Best of luck.