It's both. It shows you are one of Nyx's children if you have that on your face. It is in the shape of a moon and colored in when you are an adult but its not like in real life when it hurts to get a tattoo it just appears on their face when they are chosen. If you go to Google or pictures here you can see what they look like.
It basically means nyx has picked you special as one of her children and basically really cares about you. She cares about all her children except if you have a colored in mark it means your special but not really specailer than the rest just special. Its a moon because they sleep during they day and our night is there day and they like worship the moon thats why they have full moon rituals.
