Great whites bite each other regularly as part of normal social interaction (probably particularly around mating), that's a big part of the reason why their skin's so thick. Its toothy little friend might have come round to say hello, and left with a mouthful of tag. Doesn't necessarily mean the tagged one was eaten.
Plus, as Whiterabbit pointed out, these tags can sometimes come off on their own. Certain sharks, including the great white, are well known for swallowing floating detritus.
