This phenomenon is normally caused from moisture being drawn from the surrounding soil. This is usually caused by the stucco coming in contact with soil and extended exposure to moisture which allows the salts and minerals contained in soil and cement based products to migrate to the surface. This can eventually deteriorate and/or cause delamination of the stucco. The most common causes are: water from Sprinklers continually soaking the wall or improper site drainage (water draining toward the wall). Moisture in the soil or water sitting at the base of the wall is traveling up the wall in a wicking action; due to a lack of Weep Screed at the bottom of stud framed stucco walls.  This is frequently seen in homes built prior to early 1970’s construction boom.
This condition is common in older buildings and for this reason, the Federal Housing Authority (FHA) determined the need for “weep screed” a separation device as a remedy in new construction. For an existing structure, a separation of the stucco and the soil with the installation of a weep screed would be the only permanent solution.