Fridge Brilliance moment: Ancient vampire kid is probably used to daguerreotypes and and black and white photographs that used gelatin silver salts in their chemistry. A phone digital camera uses light sensors instead of chemistry, so it makes sense that he would show up.
(Vampires should probably also show up in modern aluminum-backed mirrors, since the myth of them not showing up in mirrors arises from them being souless/corrupted souls revealed by mirrors that were backed by silver or mercury, i.e. quicksilver, both of which were said to have mystical purifying properties.)