There were some shapeshifting Martians and Venusians in Lee and Kirby’s monster / science fiction stories, and in EC’s Weird Science comics.
It’s probably about infiltration and fears about recognising the outsider, society being undermined, manipulated and controlled invisibly at a high level, especially in those books as they were from the time of, or shortly after, the early 50s Red Menace scare and witch hunt period.
In the case of Martian Manhunter, simply a case of avoiding any paranoia and fitting in, with the ECs, a darker, more extreme, covert military / political takeover. There’s the full spectrum of intent.