The short answer
Green growth is algae, and algae need only one thing: moisture that lingers. That is why it always starts on the north side, under an overhang or behind a fence. The treatment that works kills the algae and lets the rain wash them off; the one that does not blasts them away and leaves the spores behind — and you are back where you started within eight months.
Algae are not dirt that blows in and settles on the brick. They are living organisms that colonise wherever it is damp enough and the sun does not dry them out. That explains everything you can see on your own house: the north wall is green and the south wall is not, it is worst under the roof edge where water runs down, and it comes back in exactly the same place.
The common mistake is a pressure washer. It strips the visible layer and the result is impressive on the day — but it leaves the spores, it roughens the brick surface microscopically, and a rougher surface holds more moisture. On balance you have given the algae a better bed than they had. On old pointing there is more: high pressure washes the joint out, and that is damage of a different order to a green wall.
What does work is a product that kills the algae and is given time to act. After that barely any pressure is needed: the dead layer washes off, partly at once and partly with the first rain. A facade treated that way stays clean for two to four years, against eight to twelve months after blasting alone.
Sealing is the step after that, and it is not always necessary. It makes the brick water-repellent, so less moisture soaks in and the algae stay away longer. On a north wall under trees it pays for itself. On a south wall that is dry anyway it is money you could just as well have left in your pocket — and we say so at the survey, because it is a separate line on the quote and not an automatic extra.
There is one thing you can do yourself that helps more than any treatment: make the wall dry faster. Cut back a shrub growing against it, get a blocked gutter that leaks down the facade repaired, or move a fence twenty centimetres off the wall. That removes the cause rather than the symptom.
What to remember
- Algae come from lingering damp, not dirty air — hence always the north side.
- Pressure alone roughens the surface and brings the problem back sooner.
- Sealing pays on a north wall under trees, and rarely anywhere else.