The area is shaded in winter and develops moss. It has since dried and appears mouldy. The only treatment we have tried today is high pressure hose which has resulted in black staining.
283200cookie-checkMould stains on cream paving bricks
Chemistry in the real world
Caustic soda. Put a cupful in a watering can and tip it onto the mould. HP water will then lift it. Unless it’s natural limestone, for which nowt will work