What is 異 doing in the Cantonese translation of the movie title “Us”?
異 here short for '靈異' (supernatural) or 異形 (alien)

《我們·異》 means "Us - Supernatural" or "Us -Alien" ('alien' here might refer to alien from outer space, but it can also refer to the concept of 'non-native'. In the context of this movies, it implies "unearthly or supernatural being")