You have to fix the leak and let the wall dry out first.
The algae will then die and crumble away, but you can speed it up with dilute bleach. You will probably also have a limescale deposit. You are probably stuck with that until the house is demolished, a wire brush may damage the render.
The leak has been fixed,and the type of outside walls in our estate all have broken shells as a coating.so brushing of any sort is out,as the bits will come off.
No jet wash,but have a steam cleaner and was planning on giving that a bash.
Put some diluted bleach on the bottom half of the stain and it seems to be shifting some of the muck.Im looking to move this year,so need it cleared before it goes on the market.
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