It is not yet confirmed if what is it, but it is what the Rover had found in the exploration of the planet.
According to NASA, "Curiosity's first scooping activity appeared to go well on October 7. Subsequently, the rover team decided to refrain from using the rover's robotic arm on October 8 due to the detection of a bright object on the ground that might be a piece from the rover."
NASA is attempting to identify the bit of mystery stuff to assess what impact it may have on the rover's sampling activities.
There's plenty of speculation as to what the object is. As NASA suspects, it could be a bit of the rover itself that has fallen off. It could also be part of the descent system that helped the rover land.