Where you'll find it and why
Algae needs moisture, limited direct sun, and something to feed on. On a house that means north-facing walls, elevations under tree canopy, the wall sections directly below rooflines where runoff keeps things damp, shaded concrete, and roof planes that don't get sun.
The green tint on siding, the dark streaks running down from your roof edge, and the black on your shaded walkway are all versions of the same problem.
Why scrubbing and blasting fail
Algae has root-like structures anchored into the surface, particularly on porous materials like stucco, concrete and wood. Physical removal takes off what's visible and leaves the anchors.
What happens next is predictable: regrowth within weeks to a couple of months, sometimes faster than before. People conclude the cleaning 'didn't last' when what actually happened is the algae was never killed.
What actually works
A cleaning solution — typically sodium hypochlorite based, diluted appropriately for the surface — applied and given time to dwell. The dwell time is where the work happens. It kills the organism, including the parts anchored into the surface, and then a low-pressure rinse removes the dead material.
Dilution is not a detail. Too weak and it doesn't kill; too strong and you risk damaging the surface, the finish, or the landscaping below. This is a large part of what you're paying a professional for.
Protecting your landscaping
Any solution strong enough to kill algae is strong enough to harm plants if it's applied carelessly. The approach is straightforward: pre-wet everything nearby thoroughly before starting, so foliage is saturated with clean water and can't absorb much else, cover anything particularly sensitive, and rinse thoroughly afterward.
If you're doing this yourself, don't skip the pre-wetting step. It's the difference between clean siding and a dead hedge.
What makes it come back faster
Shade you can't change, and moisture you sometimes can. Irrigation spraying the house, gutters overflowing and running down the wall, poor drainage keeping the base of a wall damp, and dense planting right against the siding restricting airflow all accelerate regrowth.
Fixing a leaking gutter or redirecting a sprinkler head often does more for how long a house wash lasts than anything done during the wash itself.