The white residue found on the exterior of your chimney is called efflorescence. It may look like the natural aging of a fireplace, but it is a sign of water leaking into your structure. If this situation is left unchecked, it could cause premature aging or deterioration of the chimney structure.

Efflorescence is caused when water moves through the chimney and evaporates, leaving behind a salt residue on the outside of the chimney. It then becomes a powdery substance on the outside of stone, block, or brick masonry. The salt deposits themselves aren’t necessarily a structural problem, but they are an indication that there is excess water present inside your chimney. If this problem is left alone, it could cause severe damage down the road.

Source: Chimney Sweep Raleigh NC | Chimney Sweep Charlotte NC