It looks like a optical reflection, but check your sensor for dust too.
Have you taken the photo with a polfilter on your lens or without a filter?
Some of the first 5D Mark III´s had problems with "hotpixels". Hotpixels normally appear in long exposures under low light conditions, but also appear if pixels on the sensor are dead. Take a few photos. If you see the whites in all photos, send in the camera for repair.