Have you ever noticed a white chalk like deposit on a brick wall or chimney? This is efflorescence - a salt deposit on the surface of masonry associated with water moving through the wall. This is usually just a cosmetic problem and can be cleaned off by light scraping. However, efflorescence can serve as a clue to an underlying moisture or water penetration problem in the surrounding structure.
The efflorescence deposits in this picture are on a chimney side just under the penetration through the roof. It is very likely that the flashing and water sealing of the chimney/roof joint are failing. This homeowner needs to take a closer look at the roof structure and shingles to see if there is further hidden damage that needs repair.
Brian Jovag, owner of Jovag Home Inspection.