This one originated as a character description, which is (I assume) why it isn’t a complete sentence.
A vampire with a rare genetic disorder that causes him to be burned by the moon rather than by the sun.
Notice that the comma after moon was deleted. This is the only required correction.
Also notice the addition of by before the sun. While it should be clear in this sentence what the author meant (the vampire is burned by the moon), sometimes leaving out a preposition can result in the sentence saying something completely different from what is intended, and you cannot always rely on context or common sense to make the real meaning obvious. The uncorrected sentence is sort of saying that the sun is burned by the moon.
(The character thus described is unquestionably a fantasy character, because there is no way someone who isn’t harmed by the strong ultraviolet radiation from the sun would be harmed by the much, much weaker radiation reflected by the moon. In fact, there are real humans who will die if they ever receive direct sunlight for more than a few minutes, and they can take moonlight without trouble. If you want to write a character like the moon-phobic vampire, you shouldn’t even try to give a scientific-sounding explanation such as ‘it’s a rare genetic disease.’)