I agree with the other posters. Sure looks like a birthmark to me and it's quite unimpressive. However, our remarks probably have little impact on the fact that you're self-conscious about it. If it continues to bother you, go back to your dermatologist to confirm that it is, in fact, a birthmark. Then, discuss options to remove it. There are laser treatments for such things. I know a girl who had them on her neck and trunk. They are no longer visible after the laser treatment.
That said, I would never entertain doing anything to make something of that nature go away especially since it's on the nape of your neck. If it were on my face? Maybe. But that's just my threshold.