#we will get to see the extent of J'onn's knowledge of other-alien cultures eventually!

[jesncin]

Were humans aware of martians before J’onn showed up to the public? Like, through satellite images or previous incursions?

Nope! While Mars is populated in my version, I have it so that one of the white martians' abilities is to be able to telepathically cloak Mars to appear as though it's an empty planet (even to technology). Think bigger-than-Wandavision-levels of sensory manipulation. I imagine Ma'aleca'andra as an isolationist planet, but martians still take interest in studying other planets (like J'onn does).

I'm still back and forth with perhaps other aliens visiting Mars and having a type of relation (peaceful or otherwise, but not outright a war) with Martians. But yeah, for the people of Earth- they don't know about Martians.