Interior or exterior?
Moisture like that is condensation of water in the air onto a cold surface. The moisture in the interior air comes from uncovered foods in the fridge, having the door open too long/often, or from air leaking in around faulty door seals. Moisture on the exterior means that area is colder than the surroundings and cold enough to condense ambient air. That typically means cold air from the fridge interior is sneaking past the door seals and chilling the adjacent exterior skin.