Efflorescence is the white, powdery or crystalline deposit that appears on concrete, brick, block, and stone. It isn't mold and it isn't paint failure: it's dissolved mineral salts that water carried through the masonry, left behind on the surface when the water evaporated.
That mechanism is why efflorescence matters more than it looks like it should. The deposit itself brushes off harmlessly — but for salts to reach the surface, water had to move through the wall or slab from somewhere. On a basement or crawl space wall, that usually means ground moisture pushing through under hydrostatic pressure; recurring efflorescence is a moisture-intrusion report written in mineral.
The distinction from mold is easy to test: efflorescence is mineral and crumbles to powder between dry fingers, and it typically appears on masonry rather than organic surfaces. Mold smears, tends to follow organic material and damp corners, and usually carries a musty odor. White fuzzy growth on a crawl space joist is a mold question; white crust on the foundation wall below it is a water-path question — and finding both together means the water path is feeding the growth.
Treating efflorescence with cleaning alone misses the point. The fix is the moisture path: grading and gutter discharge, drainage, vapor barriers and crawl space moisture control, or sealing appropriate to the assembly — then the deposits stop coming back.