I don’t pretreat if it’s just dirt or mold. oxalic acid can help remove rust stains around pool aprons.
Putting pool shock (SH) will instantly kill any vegation it touches and will destroy the O-Rings in the pump sprayer unless they’re Viton seals. A light acid coat when the concrete has dried from the washing will brighten it, makes it look nice
Let us know how it turns out.