The white scarf is called a kata. One brings a white scarf when one is visiting. It's supposed to bring good luck. It would be even better if the scarf has been blessed by a lama or holy man. These are masks which are used in some ceremonial dance to ward off evil spirits. Tibetan Buddhism differs from other forms of Buddhism, that of the Indians and the Chinese, in that it incorporates a lot of the ancient 'Bon' practices. A monk is supposed to learn the ceremonial dances in the monastery.