Are you referring to that horn-like area in back of the nails? If so, it looks like part of the plastron. The area to the left of it looks like a piece has broken off (a chunk is out of it). Has it? What does the rest of the shell look like?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
That looks very strange. It looks like a deformity growing. Since it probably impedes the swimming ability, I think you need to get a professional opinion (i.e. have a vet take a look).
If it is a deformed scute, growing at an excessive rate, then it will pose a problem. It would limit mobility and possible compromise the other scutes.