If some bricks are discoloured by, say, the overflow from a long-leaking bathroom pipe, first get the leak fixed.
Then to clean the brickwork of stains and algae you’ll need something like HG Green Slime Remover (from good DIY shops), which is perfectly safe near plants or grass but you need to use it in dry weather and when no rain is forecast for the next couple of days (not easy most years!).
Apply with a watering can or broom; no need to rinse. It takes about 36 hours to remove algae but will protect the surface against regrowth for several months.