The layer should be the waxy cuticle, and its function is mainly to reduce water loss by evaporation from the leaves.
That's why the stem of a cactus is waxy, this is because the layer can prevent the cactus from losing too much water, especially when the cactus is usually found in desert area where water is not always available.
The waxy cuticle is mostly found only on the top of the leaves, this is because sunlight shines directly to the top of the leaf instead of the bottom of leaf, plus, there are stomata at the bottom of leaves, which is like pores for water loss of the plant and carbon dioxide absorption, which is necessary for photosynthesis.
