In the comics, the Martian Manhunter is the last of his race on Mars, accidentally teleported to Earth.
While there is no definite reason known why the Martian Manhunter was eventually cut from Justice League, it is generally believed that the studio felt the movie was already overcrowded with heroes and did not need another one.
Last year, Snyder had released a storyboard shot of the scene, which takes place near the beginning of the film, before Superman’s resurrection.
On its own, it might not be clear how these images tease the existence of the Martian Manhunter in the movie as planned out by Snyder, until you realize Martha in the image is actually the Manhunter in disguise.