White staining, or efflorescence, is caused when excess moisture within the chimney structure seeps through the bricks and evaporates on their outer surfaces. In addition to being an eyesore, the discoloration indicates that the chimney is allowing water through the masonry, which will cause structural deterioration and other problems.
If your chimney is stained white, the first step is to schedule a certified chimney inspection so you can find out where the compromise exists, the severity of the damage, and the best course to take in repairing the problem.