What are these stains on brick exterior?

[Mike Knowles]

I'm rehabbing a property and there are a lot of stains/deposits on the brick exterior.  I'm not sure what they are, what caused them, or if they are able to be removed.  They are all over the exterior.  Any thoughts?

[Roy N.]

Looks like efflorescence (some old timers call them whiskers ... in your picture it's just at the stubble stage).  

It is caused by salts/minerals in the material composing the mortar, or brick itself, being leached out (by moisture) to the surface of the brick.   You can scrub, or blast, it off, but it will return.   There are products which can be applied (after cleaning) to seal the brick, but I would determine whether the moisture (water) leaching the salts is coming from the exterior or behind the brick.  {I would be concerned of the impact on the long-term stability of the brick if there is moisture trapped behind or within it}