If you've both had it since birth...Yeah, don't worry. XD! As far as I know those generally don't spontaneously become cancer. In fact, I've got two of my own that look pretty similar. Hell, I've got a huge unexplained, non-painful mark on my ribs I've never understood..
I admit that I don't know why it would change like that, but if it was a few years ago...Yeah, you're fine.