Short answer
Efflorescence is a crystalline white deposit on concrete, brick, or masonry that forms when water dissolves soluble salts inside the substrate and carries them to the surface. It is evidence of active water migration through the masonry.
Long answer
Inspectors report efflorescence as a moisture-migration indicator, not as a cosmetic finding. Common locations: interior basement walls, exterior brick veneer, foundation walls below grade. Recommended action: evaluate the moisture source (grading, downspout discharge, waterproofing) before removing the deposit, because removal alone does not address the underlying condition.