If we're all Martians now, who are the aliens?
The real aliens are those people who are so differently motivated that neither can accurately project a correct observation on one another, you cannot measure in others what is not in you, it can't be done. Therefore members of your own family can be alien to each other.
Not quite sure of your reference for this question. What I have read is that Mars was occupied by a race of giants a million years ago. That they suffered an enormous disaster which meant that a group of them escaped in a space ship, but to where? Possibly to Earth?
The Remote Viewers, originally setup by the CIA, have continued to the present day. Viewing missions have discovered dwellings disguised as land formations. And there is a possibility they are still occupied. I give a couple of links for further reading.
We are all Martians now? Where's that from?
My best guess is that it is from a speculation that the Earth was seeded by some Martian microbes who traveled to here inside a rock that was ejected by an asteroid impact.
But that is a speculation and not something anywhere close to well-established.
Even so, there is the entire rest of the Universe for other extraterrestrial visitors to come from.