Looks like a baby "elegans type" Cory. Possibly C.napoensis or C126, but no promises on what kind it will turn out to be. Looks a bit like my C126 when I first got them as juveniles, and somewhat like the baby C.napoensis I raised for a friend, they are very similar fish as adults either way. Possibly collected separately and grouped in with the C.pygmaeus as it looked similar, or it was hanging out with the adult C.pygmaeus to "blend in" and avoid predation. Either way with good food, good conditions and time it will most likely develop into an adult cory that is a bit larger than the C.pygmaeus.