This deposit is called efflorescence. When gases leak out through a defective liner and onto your brick interior wall they deposit salts on the surface of the masonry. Water then dissolves and transports these salts through the joints and mortar to the exterior of the chimney where you see the white deposits. So these are the telltale signs of leaks in your liner and water problems in your chimney system. Repairs should be made immediately to prevent further deterioration of your chimney.