A benign tumor made up of newly formed blood vessels is known as

This is called a hemangioma.

A hemangioma is a benign, tumorous, and localized growth of newly formed blood vessels; particularly in infants and children. Small hemangiomas are colloquially known as birthmarks. Hemangiomas do not cause clinical issues although the most common of which is aesthetics. These tumors rarely becomes cancerous (hemangiosarcomas) and medical treatment is not indicated.