Efflorescence usually occurs immediately after staining. Efflorescence is a white crystalline, powdery or often fluffy/fuzzy deposit on the surface of masonry materials like concrete, brick, clay tile and in this case plaster. It’s caused by water seeping through the stucco wall. The water dissolves salt as it moves through the wall and then evaporates leaving the expanding salts on the surface. Efflorescence is often the root cause of blistering.