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It doesn't appear to be shedding. What is it?

Most likely a bite wound in a state of healing. Hard to say from the pic.

I couldn't get a better quality picture. Nommy is very shy. I doubt His tankmate Chibi would have bitten him. They are never aggressive towards each other. Could it be his neck rubbing against his shell? He gets startled easily and tends to pull his head in fast when that happens. I'll keep an eye on it and make sure it isn't growing.

If the nuchal scute sticks out a lot, then it could cause a problem, but that doesn't look like it's an issue from the picture.