Papa suggests that 'rocks' with square holes in them may not be rocks at all but the effects of westhering on machinery of some kind with square openings for the machinery to fit together. However, I dont see why the square holes would remain square if the outside has weathered. Most hard edges get softened over time - at least on Earth they do but maybe the weathering is different on Mars..
However, if you look at where it is in relation to the Rover, it looks to me as if it could be a RAT effort on the rock. Normally, the RAT makes a single roughly circular patch but this isn't so I dont know if it is caused by that. The "shading" around the outside makes it look like that might be sandings from the grinder. I dont see it as anything off the surface of the rock, but maybe it is. Take a look at other RAT places whre they have done sampling and smoothing and see if you think this is what it might be.