Zack Snyder added Martian Manhunter to his cut of Justice League during last year's final round of reshoots, and while he appeared in two scenes (his transformation outside Lois Lane's apartment and that final meeting with Bruce Wayne), it was only supposed to be one.
The filmmaker wanted to use John Stewart in the epilogue, and even filmed the character's scenes with actor Wayne T. Carr. Unfortunately, Warner Bros. stepped in and pushed for that Green Lantern not to be used because the studio had plans for him elsewhere. That's probably the Green Lantern Corps movie, with the thinking being that having the hero debut in the Snyder Cut would have lessened the impact of his eventual arrival on the big screen.
As a result, Martian Manhunter replaced Stewart, and Carr's scenes were left on the cutting room floor.
Now, though, the actor has shared some photos from his time working on Zack Snyder's Justice League, and as you can see, he really did shoot Green Lantern's scenes on the director's driveway! Unfortunately, as special effects were going to be used to create John's costume, we only see a mo-cap suit here. We also probably shouldn't bank on his scenes ever being finished and released.
However, should Snyder get to make his sequels, then perhaps Carr will be able to reprise this role!
Regardless, you can check that out his Green Lantern thanks to some concept art from the movie.