As mentioned you used too much pressure so you disturbed the oxidation.
Surfactant helps the SH stick to the siding which makes for longer dwell times. It also helps break water surface tension which can make the SH more effective. If the algae or mildew isn’t just melting off you might not be putting a strong enough mix on it. A High Draw chemical injector is what a lot of us use. The black or green algae should turn brown and then easily rinse away. As far as pressure imagine you’re using your garden hose at home and watering the flowers using the rain setting on the nozzle. That’s about all the pressure you want. Maybe a touch more but nowhere near 400-500 psi. You want to use less than 100 psi. If not, you’ll disturb oxidation and risk blasting water back behind the siding.
