That white, powdery stuff you see formed over masonry, stone or concrete walls is what’s called efflorescence. When these types of outdoor materials effloresce, it means that water has seeped through them (from one direction or another) and has come out the other side bringing with it certain types of salts. The water may evaporate, but the salts just dry up and crystallize giving you the undesirable efflorescence.

Can you remove or clean off efflorescence?
You can remove efflorescence relatively easily with high water pressure (spray) or with acids such as muratic acid. Products such as CLR or Lime-Away can work too if used with water and a brush.  A lot more work though! Sometimes efflorescence can be prevented by eliminating or reducing how much water can come through the poruouse materials that the white salt crystals are forming on. For example, you may have luck applying a concrete or stone sealer to the surfaces (after you clean the efflorescence) so that water doesn’t penetrate all the way through to the surface. If effloresence is caused by groundwater or a sprinkling system that wets the earth on the other side of a retaining wall regularly, you can place a water barrier (plastic liner) between the soil or dirt and the wall itself. You might also want to improve the drainage in that area too. These will keep moisture from seeping through the wall and evaoporating, leaving behind efflorescence.