It's not a specific disease but rather the result of a disease process or health condition in your cat. Whether it's mild or severe, it needs to be treated.
Like every veterinarian, I evaluate the mouth and gums of my patient whenever Im performing a physical examination. I expect to find nice pink gums, and most of the time, I do. Occasionally, however, Ill note with dismay that the gums are pale, or even white. This finding immediately sets off alarm bells in my head: This cat is anemic. The task of making a diagnosis now begins.