What is on mikhail gorbachev's head?

It's a birthmark.

This type of birthmark is known as a "port-wine stain" birthmark. The formal medical term is nevus flammeus. It results when insufficient nerve fibers in a particular area of skin allow more blood than is the norm to flow through capillaries in that area. This extra blood flow causes the dark red blotch that is the port-wine stain.