Efflorescence
A white powdery or crusty deposit left on a concrete surface when water moving through the concrete carries dissolved salts to the surface and evaporates.
What does efflorescence mean?
Efflorescence is a symptom of moisture movement, not a defect in the concrete itself, so the durable fix is controlling the water rather than cleaning the salt. Fresh deposits often wash or brush off, while older crystalline deposits may need mild acid or specialty cleaners. Sealing a wet surface without addressing the moisture source can trap it and cause the sealer to fail.
In practice
White staining on a basement wall or the edge of a paver patio means water is moving through, and the sealer aisle is not the answer.