"He is not familiar to me."
Translation:Αυτός δεν μου είναι οικείος.
Οικείος is very formal. It is better to use the adjective: γνωστός for everyday speech
Why does the μου precede the verb? αυτός δεν είναι οικείος μου was marked wrong. I guess it would literally mean "he is not my familiar."
The short versions of pronouns (accusative με σε το etc. as well as genitive μου σου του etc.) come before the verb.
If you want to use them elsewhere, you'll need the long version: σ' εμένα "to me", for example.