This indicates water infiltration, leeching salts out of the stone work where they condensate on the surface. Somewhere there is a water leak. Past or current. As an aside, the chimney has not gone with out maintenance, we can observe what appears to be new work, with the chimney masonry cap and flew cap. Nice to see.
Leakage can occur if the flashing leaks water.
This condition can be seen in the photo above right. Roofing life expectancies can vary depending on several factors. Any estimates of remaining life are approximations only. This assessment of the roof does not preclude the possibility of leakage. Leakage can develop at any time and may depend on rain intensity, wind direction, ice build up, etc.