redearslider.com

Phong has a few white spots on his shell, which were there since I got him. Could it just be mineral deposits or something I should worry about?
HUGE PIC

Welcome. I doubt they're mineral deposits, but if you want, you could try rubbing a little white vinegar on the areas with a q-tip. If it were mineral deposits, the white would come off. A touch of fungus is also possible from prior living conditions. If the white is "in" the scutes (can't be rubbed off), it's likely damage to the shell from living conditions.

Give your little RES (cute, by the way) a good habitat and diet. As s/he grows and sheds, the white areas should disappear.