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I noticed this a few weeks ago after I took Ralph outside (and he ran into a rock ) I'm not sure if it was there before, it may have been and I just hadn't noticed.
Anyhow, there are two white spots on the edge of his shell near his neck. They aren't soft, and there is no smell to them. I've been keeping a close eye on them, and they've niether spread nor shrank. They are also no rougher than the rest of the shell. They're basically the exact same as the rest of his shell, only white. I'm wondering if this is the start of shell rot? Or perhaps a scrape from running into the rock? Or perhaps just a loss of pigment?
His behaviour is completely normal, and he's basking and swimming fine... (He's just under 7inches, if that helps any)
Here are pics:

The pic is a bit blurry, but it seems a layer of shell came off to reveal what is affecting the rest of the shell. Can you scratch off some of it?

He's shedding his scutes right now, which is why the rest of the shell looks lighter I think...
It doesn't scratch off or anything, and there's no difference in the height between the white part and the normal part...

Is that where the shell hit the rock?

I think that's where he hit the rock (He sort of ran into it head first! =S)

I tried to take some better pics today, but they didn't turn out any better than those ones. However, I did notice it seems to be going back to normal. It's got a greenish tint under where it was just white before. So I'm thinking it was just a scrape or bump or something. Maybe like how we sometimes get white spots on our nails? I'll continue to keep an eye on it, and will post if it seems to be getting worse