there's always questions on this item & there's an article in this month's issue of ' builder ' magazine
efflorescence = surface deposit of whitish powdery material of mineral salts. salty stuff remains as carrying water evaporates. sources can be mortar sand or deicing salts rising from the ground. when 1st seen, its usually soluble in fresh wtr & can be pressure wash'd off,,, after time, it may combine w/CO2 & form an insoluble carbonate,,, time hardened deposits may come off only w/acid which carries its own risks.
prevention is a combination of things: 1, avoid salts in sand & mortar; 2, shelter brick from rain & groundwater, isolate brick from cmu walls, 3, apply a breathable crystalline waterproofing solution ( water-tight coatings can trap salts below the surface & expanding salt crystals can blow off the brick surfaces ); & 4, vapor barrier behind brick OR install proper waterproofing,,, also applies to cmu's
apologies for post length but link didn't work