A choroidal nevus is a freckle in the back of the eye. There are no obvious symptoms of having one—they can only be seen by an eye specialist.
There is a slight risk of a choroidal nevus becoming cancerous, like any mole or freckle. This is why your optician will help you pay close attention to it.
A choroidal nevus is essentially a flat, coloured area that can appear on the back of your eye after birth. It is not known for sure what causes them to develop.