Sweet little duckling. It may have a palate defect inside the bill as well as the visible deformity. A simple reason for it not growing as fast as the others might be that it is not getting as much food down. Keep an eye on its intake -- water as well as crumbles. But the little one may well grow and live normally with time. I have had two ducks with deformed bills -- both traumatic rather than congenital. They did well as adults feeding from deep pots and "catching' their peas in water in a deep saucer.