How do I get rid of moss on a concrete foundation?

A dilute Bleach/water mixture will kill it... but unless you resolve the conditions that encourage it (I'm guessing its in a lot of shade) its going to return.

Vinegar and saturated salt solution works well or why not pressure wash. However, as Smoothy said, you need to find out why the moss grows there..

At a guess, lengthen the drain line. Mosses need a lot of water. Then clean it off the stone. Smoothy and Ma0641 have both offered good cleaning solutions. The pressure washer is an even better idea.