Algae must be removed with a high pressure rinse followed by treatment, which will keep it from coming back. Algaecide, or a solution of bleach and water, will do the trick.
Mix 1-part bleach for 4-parts water, and scrub by hand using a brush. Never run bleach through your pressure washer, as this will cause damage to your machine.
Let the cleaning solution settle in for 10 minutes, then thoroughly rinse with clean water.
Taking care of heavily encrusted stains may need the extra power of a turbo nozzle. A turbo nozzle will help you blast away pesky build-up with ease, letting you work more effectively.
Turbo nozzles pair a high pressure stream with more coverage by spinning the stream very rapidly. It enables you to efficiently strip hard-to-remove stains from the concrete/plaster sides of your pool with less likelihood of doing damage.