Dandruff is actually clumps of skin cells that have sloughed off your cat's body. These clumps can look like scales or flakes. However, not all scale is due to dandruff and it is a good idea to get your dandruff-covered cat checked by a vet. A vet check up is recommended to make sure your cat is not suffering from an illness that is impacting its general health and body condition. These could include diabetes, overactive thyroid glands, arthritis, or seborrhea. If your cat has one of these conditions your vet will make suggestions for treatment. One particular cause of confusion is the skin mite, Cheyletiella yasguri. This mite feeds on sloughed skin cells, which causes more scale to develop, and the mite itself mimics the appearance of scale. Indeed, it has the nickname "Walking dandruff".  Your vet can quickly tell if Cheyletiella is a problem by knocking samples of dander onto a microscope slide for examination under high magnification. If the mite is identified then your cat needs treatment with a product containing fipronil. This treatment is generally given every other night, with 3 treatments minimum, in order to kill the mite and resolve the apparent dandruff.

Summary:
Identify dandruff. Take your cat to its vet. Rule out infections that may present as regular dandruff.