Congenital moles on the left arm in a 20-year-old woman. Also known as a pigmented, melanocytic, or melanotic, naevus, this skin mark is caused by an abnormality or overactivity of the melanocytes, the skin cells that produce the pigment melanin. Moles vary considerably in size and appearance. They are unusual at birth (congenital) and tend to appear in childhood and adolescence.