Why does my baby Red Eared Slider have a discolored shell?

I wouldn't worry so much about a little discoloration on a RES. What I would do is change the diet from fruits and veggies and Reptomin to what the turtle would find in the wild. For example, guppies or minnows..dump a dozen or so in the tank with the RES and let him exercise while he learns to catch live food. Also, drop a worm or two in the water for him to find while he scavenges on the bottom. Try to think of how this animal would act in the wild and then accommodate that behaviour. You will find your turt will be healthier in the long run.