With our damp and warm climate, algae growth on external surfaces can be a problem. I generally just use a power washer every few years to clean the sheltered concrete areas where it can be a problem. However I discovered that the power washer was unable to remove the algae for my brick garden walls, probably as the brick was more porous than the concrete.
However lightly scrubbing the walls using a dilute solution of water and bleach provides an excellent result with little effort, see below.
The rendered wall below simply had the water/ bleach solution sprayed on rather than scrubbed and returned great results also. It is also far cheaper and more effective than the marketed brands e.g. fungicidal washes, Jarvis fluid etc. Be careful though and don’t add too much bleach as it will probably kill the surrounding grass and plants and don’t let it come into contact with skin, pets, clothes etc.