Just a guess: That’s most likely ice from when a comet went “splat” (creating the crater), part of it “bounced,” and that part went “splat” again (creating the smaller crater inside the bigger one).
Any ice would have been instantly vaporized on impact.
I think ice would have disappeared by sublimation. Unless it’s continuously replenished.
It also shines when this side is dark. Its most likely lava.