Hi
Choroidal naevi are pretty common occurring in approximately 8% of the population. It is debated whether melanomas can come from pre existing choroidal naevi or whether they are a completely separate entity. The risks and suspicions associated with these pigmented lesions are generally taken from their size, whether they are raised, if they have surface orange pigment (lipofucin) and proximity to the optic nerve head.
It is thought that possible progression from naevi to malignancy is extremely low at approximately 1 in 10,000 I think but I would presume if you where thought to have a suspicios lesion it would be monitored closely and sometimes this is done at specialist ocular tumour centres such as Sheffield, Liverpool and moorfields in London.
Treatment can vary depending on the location and size of the lesions if there were need for it and this is best discussed with the specialist dealing with it.
Hope this helps