Is it harmful? NO!
Per the Webster Dictionary, efflorescence is the act or season of flowering; a. blooming forth, to become powdery, wholly or in part, and lose crystalline structure through loss of water of crystallization on exposure to the air. To become covered with a crust of saline particles left by evaporation or by chemical change.
What does this mean to the basement with white powdery efflorescence on the lower walls and on the floor?
It means that a chemical change has taken place in the cement mixture used to pour your basement. As the cement concrete continued to dry out, the saline (salts) in the water crystallized and deposited on the surface of the concrete.
Is it harmful? NO more than table salt.Is it mold or mildew? No. Mold and mildew are living organisms that live off of (eat) plant products (i.e., paper on sheetrock.) Salt is a mineral (rock.)
Best way to get rid of it; wash it with warm soapy water.
However, it will leach through again as time goes on. Use Kilz® and paint over it, however, salts will eventually deteriorate paint too. If the home or building was constructed in a region that has a higher saline content you may see an increased amount of efflorescence in concrete.