They really should have been separate for a while before you put them together in your tank just to make sure that turtle didn't have some illness that can infect your other turtle as well. The reason is that turtles are GREAT at masking illness so you don't know they are sick until weeks after you get them. On this website, the Mods recommend isolating a new turtle for 90 days when you first get them before putting him in with any other turtles just to make sure.
Is there any way of getting a clearer photo of the turt's shell? Are the spots soft, yellow, or do they have a smell to them? Are they peeling in the spot area only ("pitting")? All baby turts start off with softer shells than the larger turtles, but I am more concerned about the spots being soft.
What's the water temp of your tank? Do you have UVA AND UVB lights in your tank or just UVA? Do you know the brand of the lights? Just trying to get to the bottom of things!
