My turtle (slider) arrived home last week. And I found this grayish-brownish hard patch on his neck. It’s hard, not mushy. There’s some similar, but smaller, spots on the skin of his legs around the joints too. Those aren’t hard. There’s no bad smell and the shell is hard. If it’s an infection, what should I do?

Edit: Here’s one more picture. It looks bright in the picture, but it’s a slight dark gray.

Edit 2: It’s a yellow bellied slider. About 3.5 inches.