There are two small very straight lines on the lower right of the object that hints that it's man-made. Really looks like something just fell off of the rover, rather than was knocked there from the ground nearby. There's almost no disturbance of anything else in that area which would be expected if that object rolled or was pushed there. Looks like it fell down on top.
The NASA page has much better copy about the object:
Much of the rock is bright-toned, nearly white. A
portion is deep red in color. Pinnacle Island may
have been flipped upside down when a wheel dislodged
it, providing an unusual circumstance for examining
the underside of a Martian rock.
From this, I get the impression that the other rock you see in the photo is actually bright white as well, but is covered by red martian dust. This is probably the most interesting thing about the whole photograph.
