From a spiritual perspective an alien thought suggests one that goes against the grain.
An alien being may be a manifestation of evil or, as something different and not yet understood, the occult.
Psychological / emotional perspective: To recognize that you feel alienated within a dream suggests that there is the potential in everyday life for experiencing oneself – or a part of oneself – as not belonging.
To feel alienated from the dream scenario is the realization of being different from others in the way we live our lives.
Material aspects: To dream of an alien being suggests that there is something unknown and frightening which needs to be faced. We have never encountered the strangeness of the being that appears in our dream and we must handle whatever happens.