The condition you describe sounds like idiopathic non-inflammatory leukoderma. There are seem various names for this condition including pinky or fading syndrome. The closest equivalent condition in human terms is vitiligo. Vitiligo also affects other household pets and some livestock.
I know of a part-Arabian who had this condition and lived to a ripe old age. The skin around his eyes was pinky white and I remember that the shape of the pale skin area changed somewhat over time. The shape and size of the vitiligo can spontaneously change shape and size or disappear completely. Vitiligo can affect the muzzle, face and genital areas, affecting any area of the horse's body that is thinly haired. There is no treatment that I know of. So what you are seeing is probably only a cosmetic condition and not harmful to the horse.
Some maintain that vitamin or mineral deficiencies can cause vitiligo, so you may want to check your feeding routine and add a supplement to make up for any possible shortfalls. Vitiligo is also considered hereditary and more common in some breeds.
There are several natural remedies that you can try. Wheat germ and cod liver oil are the most inexpensive and easily found. Simply apply a thin film of either over the area. You can also try homeopathic and herbal remedies.
If however, you are at all concerned or there appears to be tearing, swelling, cloudiness or other unusual symptoms in the eye or eye area I would recommend you call a veterinarian. Never take a 'wait and see' attitude with eye infections or injuries. If infection sets in, permanent damage may occur.
Not all white patches are vitiligo. If the patches appear in the saddle or girth area they be scars where saddle sores or girth galls have been. White patches in the saddle area can also indicate where a poorly fitting saddle is causing constant pressure. The areas may not become raw as can happen with saddle sores, but the pressure can cause a restriction in the blood flow, damaging the hair follicle, impairing pigment production and causing the hair to grow out white. That is why attention to good saddle fit is so important.
Scarred areas on any part of the legs or body can grow white hair. White patches can indicate an old injury, possibly from an accident with a fence or stall wall, kick or bite from another horse or anything else the horse can injure itself on. My own horse has a white strip on her knee where she was injured in a trailer accident. Frost bite and sunburn can also cause white scars.
Some breeds like Appaloosas, Clydesdales and Arabians sometimes 'grey out' gradually developing flecks and spots over their bodies. This is a normal coat pattern. Spots that just appear for no discernible reason are called 'birdcatcher spots'. These spots can vary in size and may include a few hairs or coin sized ares. They may appear and disappear, usually when the hair coat sheds out in spring and fall. A forum member describes her experience with birdcatcher spots.
There is one disease called Hyperesthetic leukotrichia that is very painful for horses. This usually appears as white hair over the central back area. If the area of whitened hairs appears to be painful call your veterinarian for advice.