I agree with jscrick. Either there is some underlying heath issue, or it's a healed/healing injury. Could be caused from fire, trauma, or an infection of some kind. Or it's possible the turtle has something like age induced pigment loss (simlar to the skin disorder that some people get as they get older). It's not a genetic mutation, though. I'm willing to bet a health issue of some kind, either in the past or on-going.

Katrina
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0.1 Iguana - Tiffel
1.2 Eastern Mud Turtles - Fred, Ethel, Edith
0.0.1 Giant Mex. Musk Turtle
Foster turtles: More than I'd like the husband to know about.