For the most part vampires have to stay out of the sun. There are some stories where they can walk in the sun with various consequences, some minor, some extreme. But what if a vampire who lived in a world where vampires had to stay out of the sun was fine in the sun. Instead it was the moon that he or she had to avoid?
Personally, I think that would be more difficult. The sun is easy to pay attention to, but the moon is different. Sometimes both the sun and the moon are up at the same time and for the most part people don’t even notice.
A vampire with a problem with the moon would be able to be out during the night and day depending on the cycle of the moon. They might have to stay inside for an entire day because they overlapped that day. It would make scheduling a job very difficult. There would be no nine to five job for them, but there would also be no night shift for them.
I think a character with this problem would be quite interesting. The moon just reflects the sun’s rays, so what would make a vampire get burnt by the moon?
The prompt calls this a rare genetic disorder, but how rare? Are they they only one with it or are their other vampires with it? Does this have to do with who they were and their genetics before they were turned or did something bonk up when they were turned?
How do you think a vampire community would view someone with this situation? Would they be pitied, envied, or cut off? What kind of life would this character have?