Aliens were a pretty big deal during 'Invasion', the four series crossover between the Arrowverse shows, because they showed up on Earth-1, where aliens are not a part of regular life. Supergirl is not set on Earth-1 however, and on that show aliens are everywhere. Two main characters are not from earth, Kara is from Krypton and Hank/J'onn is from Mars. The entire world is aware of the existence of aliens, to the point where laws are being set up by politicians to protect them.
That being said, not all of the aliens who come to National City are good guys. Supergirl and the DEO have fought various alien species who had no regard for human (or extraterrestrial) life, or even some trying to take over the world. Currently, there is a ship full of aliens in search of Mon-El and The Dominators from 'Invasion' are scheduled to appear.
And there is another hostile race on the way. In the synopsis for upcoming episode 'The Martian Chronicles' there is a White Martian headed for National City:
WHITE MARTIANS ATTACK THE DEO — Armek (guest star Terrell Tilford), a White Martian, descends on National City intent on taking M’gann (guest star Sharon Leal) back to Mars to face her punishment as a traitor. Hank (David Harewood) and Supergirl (Melissa Benoist) determine the best way to keep M’gann safe is to bring her to the DEO. However, when it is revealed that Armek shape-shifted into M’gann and is now loose in the building, the team realize he could be disguised as any one of them. David McWhirter directed the episode written by Gabriel Llanas & Anna Musky-Goldwyn.
This sounds like a potentially exciting episode. Most of the main cast works or lives at the DEO, meaning Armek could disguise himself as Hank, M'gann, Kara, Alex, Mon-El, or Winn. Not to mention any of the other employees of the DEO who the audience is less familiar with. It could be fun to see him disguise himself as some of the regulars since it would mean the actors could get the chance to play someone else for a few minutes. If Hank or M'gann also disguised themselves to fight him, it could be a pretty crazy hour of television.
Seeing J'onn confront one of the alien race who committed genocide against his people and killed his family should also be interesting. While he's experienced some of that in his dealings with M'gann, she ran away from her people because she knew what they were doing to Green Martians was wrong. This time, J'onn will be facing an active participant. This could also help strengthen their relationship, which is supposed to be an important element of the rest of the season.
Supergirl returns to the CW at 8 p.m. on Monday, January 23 with 'Supergirl Lives'.
Source: The CW