Gold spots on his shell

Hi there. I have a 6-month old DBT who has a few gold spots on his shell. He’s had one for almost the whole time i’ve had him (and it hasn’t gotten any larger), but i’ve just noticed one or two other spots. They’re very small(~1-2mm diameter), irregularly shaped, and metallic-looking in the water. I originally thought the first one must be a nick, but the other spots really don’t look like nicks. Besides, there’s never really been anything for him to chip his shell on. He’s quite healthy and robust, has a good hard shell and healthy appetite and everything else. I keep his tank at a spec grav between 1.018 and 1.020 and feed him reptomin. If anyone has seen this, could you let me know? Thanks! P.S.-if anyone has an idea of what live food i might be able to feed him, I’d love to know. I’m planning on releasing him this summer, and i’d like get him accustomed to eating live stuff. -annie