Milk pimples?

Amber • 20 years old, mumma to my beautiful boy Oliver and expecting my second in Jan 2018 👶🏼
Does anyone know what caused milk pimples on baby's face and chest and how to make them go away? My bub is breast fed with the occasional top up of formula and he doesn't have lots of milk pimples but has a couple on each cheek and a few around his head and chest