Natts usually have the same humerall spot that Cariba have, although you definately cannot see it as clearly. It is normally very faded and barely noticable, although it is there if you looking for it. George had pictures of tons of Natts with obvious humeral markings on them during one of his trips.
Melanophores are pigment (black) cells which cluster to make spotting other other attributes of your piranha.
This is harmless. Your fish is not P. cariba and it is atypical to find a faint blemish (humeral spot) on P. nattereri. On your fish the blemish is on the opercle, hence, melanosphore pigment (blemish).