I am partial to 'Outsider'.
It may be the best synonym for 'alien' you may ever find that evokes a sense of 'not of this world' or 'from beyond' within the context of the genre. Best of all, it is quick and easily identifiable.
Singular, subjective common nouns such as 'outsider' which are simultaneously concrete and abstract are much more succinct than adverb prepositional phrases such as 'from beyond'.