Since Lynda Carter's first appearance on Supergirl as the President of the United States, we knew that her character was an alien. However, her true identity could be more than just a threat. Is the President a White Martian on Supergirl? We only got a glimpse of her true form, but this doesn't bode well for J'onn J'onzz or America.
Have aliens really been on Earth long enough to get involved in politics, or is this alien shapeshifting as a real person? The true President Marsden could be locked in a trunk somewhere like Mad-Eye Moody. Is this why she didn't want the DEO to intervene in the bounty against Supergirl? What is the President planning?
Personally, I hope that this is a red herring and that Marsden is a "good alien," so to speak. What kind of message would it send if a country working towards rights for alien refugees had a genocidal alien pulling the strings? A not great one, I'll tell you that. Just the fact that this information is being dolled out mysteriously on Supergirl is a good way to toy with our expectations and judgements. Xenophobia applies to fictional aliens as well. We know that most aliens on Supergirl aren't villains. Even M'gann was a hero.
Plus, we haven't seen too many White Martians on Supergirl, so it's hard to confirm her species. The best descriptor I can come up with is that her face looks like a baseball. However, there aren't too many shape-shifters on this show, so the odds are decent. Even if the President isn't a White Martian, she's still keeping an important secret from the American public.