crumbling exterior plaster
The blisters in the photo are due to moisture (water vapor) being forced out of the from the opposite side or from the ground. Moisture permeates through the plaster in the path of least resistance, such as a fissure or hairline cracks. If the surface has been painted the water will actually blister the paint as it continues through the wall. The powdery residue you notice is a combination of minerals left behind when the moisture evaporated and whatever effluence it picked up traveling through the plaster. To solve the problem first eliminate the cause of the moisture. This could be any thing from a wayward sprinkler that splashes water against the walls surface to gutters saturating the ground that is against the wall. If the wall is in contact with the soil, excavate to the footing clean the surface and when dry apply a concrete sealer like Dryloc. To repair the blisters wire brush the loose plaster scrape any flaking paint, apply a concrete adhesive to the surface (not critical to the repair, but it helps), and apply a plaster patch as needed. It will blister in another location if the water source isn't corrected.