Where the salt actually comes from
Concrete holds calcium compounds left over from the cement, and water moving through the slab dissolves a small amount of them the same way water dissolves salt in a shaker. As that water reaches the surface and evaporates, cold air, sun, or a dry stretch of weather pull it out fast, and the dissolved minerals stay behind as a white, powdery film. It is not dirt sitting on the concrete, and it is not something left off the surface during finishing. It is a byproduct of moisture moving through the slab, and any slab with enough water passing through it can produce it.
When it shows up on a new slab
New concrete carries the most water and the most unreacted lime, so a young slab, especially a stamped or colored one, is the most likely to show a light haze in its first year. It often washes off in the next hard rain or fades on its own as the slab finishes curing and the excess lime works its way out. That first year bloom is common enough on new work that we mention it up front on colored and stamped jobs, so it does not read as a defect when it shows up.
When it means water has somewhere to go it should not
Efflorescence that keeps coming back on the same spot, year after year, past that first season, usually means water has a steady path into or through the concrete from somewhere. A slab poured without a vapor barrier under it, a foundation wall without drainage on the outside, or a crack letting water track through are the usual sources. In that case the salt bloom is not the problem so much as the messenger. The real fix is finding where the water is coming from, not just brushing the surface clean.
Getting it off, and keeping it off
A stiff brush and plain water clears most efflorescence, since it sits on the surface rather than in it. A mild acid wash handles a stubborn film, though it is not something to do on colored or stamped work without knowing what the acid will do to the color. A penetrating sealer, applied to a clean, cured, dry surface, is the longer term answer, since it reduces how much water and dissolved mineral reach the surface in the first place.