Invisible rock
The rock is still there in the lower picture, just in that particular waveband it appears dark. Youve lost the original filenames, but if you go back and recover them you'll probably find that that is an IR filter on the right "eye". They often do this with non-sulphate rocks.
The rock is clearly different in its response than the substrate - it's fallen into the crater from a higher level, or might even be a secondary fragment from an impact elsewhere.