The layer that forms on top beverages consists of fat. Such a layer is formed only on top of hot liquids because in the unheated milk the fat globules are surrounded by a thin film of protein, phospholipids and neutral fats.
When the milk is heated, this thin film breaks, making the fat globules free. These then coalesce on the surface which then come to contact with the air and forms a layer. Formation of this layer can be prevented by beating of string the heated milk continuously, as the process of homogenization increased the surface tension of the milk. But when the heated milk is allowed to cool and left undisturbed the layer formation occurs. In this condition the surface tension gets lowered and the freed fat globules rise to the surface because their specific gravity is less than that of the aqueous portion of the milk. Also black tea or coffee will not be able to form any such layer.