反差萌 comes from Japanese anime culture (ギャップ萌え, gap moe). It describes the charm of someone showing an unexpected side — like a tough guy being gentle with animals, or a serious professor making silly jokes. Very popular in fan culture.
usage
Can describe real people or fictional characters. 萌 here means 'cute/adorable' (from Japanese 萌え). The bigger the contrast, the stronger the 反差萌 effect.
