Yup. Dandruff is most often seen in the mane and/or tail, but can spread over the entire body. It's caused by all kinds of things - allergies, dehydration, hormonal changes, stress, mites, or just, you know, a genetic propensity to a dry skin. It's sometimes seen in show horses that are bathed too often.
It's treated by adding fat to the diet - usually in the form of corn oil or flax seeds. Extra grooming can also help. And, of course, you can use an anti-dandruff shampoo (not a human shampoo, the PH is different, and using human shampoo on horses can in fact cause dandruff). Some people also use vaseline or mineral oil to moisturize the horse's skin, and there are various lotions made for this annoying little problem.