Many horses will shed hair around their muzzle and eyes during the spring and summer. Horses out on pasture all the time can lose hair around their lips and eyes due to increased moisture on the grazing in the mornings. If there is no scabbing or crusts on the face, your horse is probably fine. If scabbing or flaking starts to occur, contact your veterinarian, since that could be a sign of a fungus problem.
Also, white-faced horses may show more irritation on their faces than dark-faced horses. White-faced horses can get sunburned on the white areas, which can result in loss of hair. You can use a sunblock on those areas.
