Depending on how bad? you may be able to spray Clorox cleaner + bleach on it set it set for few minutes and wash it off with hose. I had to scrub few spots that was darker.
https://shop.clorox.com/products/clo...cleaner-bleach
Or you can just mix 10% bleach water (wearing gloves) and scrub it in and rinse off.
If you got long wall and years old mold build up bleach water and pressure washer probably only option?