What causes red brick to turn white on the surface? This was once explained to me, but all I remember is that it comes from buying cheap red bricks.
That would be soluble minerals from the brick, or from groundwater that soaked its way up into the wall, leaching out. The brick gets wet, dissolving stuff from inside and leaving it on the surface as the water evaporates. If the makers used good clay and fired the bricks properly, there won’t be too much soluble stuff and it will be bound pretty tightly anyway, though any brick will show the effect. Concrete walls do the same thing, especially in basements where the groundwater comes through.
See if the bricks are in contact with wet soil, especially if the soil is very alkaline, as it is in the prairies and quite a few other areas.
The process is called efflorescence and is due to salts leaking from the brick. It’s not necessarily due to the brick itself, the salts can come from the mortar, or even the ground. (article)