I've had a couple appear and disappear with no ill effects. Not all are cancerous....
But let me tell you - melanoma is not to be trifled with. Caught early, the cure rate is good. Left, the cure rate is dire. And melanoma is on the increase.
Can I beg, I really mean beg, that if you have the slightest, tiniest thought that a mole or a freckle or a patch of pigment is changing, you go to your GP? Immediately. Don't wait, don't think "oh, it's nothing" or "I'll be wasting their time." It can present as tiny moles, or changes to larger ones - go see your GP. When I lived in NZ it was common to go for a whole body check once a year.
A few years back I worked on a drug trial for melanoma. So many of the patients were women in their 30s and 40s, I found it extremely disturbing.
I'm fair and freckley. I'm totally anal about safer sun exposure (you need a bit, but don't ever redden or burn.)