J'onn explains to Alex, Kara, and the others the Martian history of Phobos and Deimos. In addition to the names of Mars' moons, Phobos and Deimos are, in mythology, the sons of Ares, the Greek counterpart of Mars, the Roman god of war.
Phil LaMarr who portrays Malefic J'onzz, voiced Green Lantern (John Stewart) in Justice League (2001)/Justice League Unlimited (2004).
Around 31.5 minutes, Supergirl is fighting J'onn Jonzz's brother (pretending to be Alex). "Alex" uses a pick-up truck's side mirror to reflect Supergirl's heat vision back at her. This tactic is an homage to the same tactic used in Superman II (1980). Superman (Christopher Reeve) uses a truck's side mirror to reflect General Zod's heat vision back at him during a climactic fight scene.
For the scene where Malefic tries to trick Alex by impersonating Kelly, actress Azie Tesfai had prepared a whole list with ideas on how to play the fake Kelly, which included 'moving unrealistic', 'not blinking', 'imitating emotions' and other weird behavior that would make Kelly seem creepier. Tesfai knew she had made the right choices when her performance creeped out Chyler Leigh.