First, the moss is of no concern to the concrete wall other than any appearance considerations. You don't need to remove it unless you want to.

If you must remove it, power washing (or a strong stream of water from the hose) will do the trick. If the surface is small enough to be manageable, you can also use a paint scraper to scrape it off.

Avoid using any of the bleach or vinegar solutions posted online. They will kill or seriously damage any desired vegetation present.