What is efflorescence Efflorescence are white, powdery type of crystallized mineral salt that appear on concrete, stone, and stucco, in the form of ugly stains that often ooze, flake, and peel off.  They are also often seen on unfinished basement walls or growing inside the surfaces a home’s crawl space.  The worry, of course, is whether or not these “petri dish” stains are harmful mold [...]