The picture's so big it's hard to get the proper perspective, but I'm thinking this is on the side of the carapace? Nothing needs to be done about it. It's not fungus or shell rot, as those almost always are caused by the tortoise sitting constantly in dirty, wet substrate, poop or pee, hence it will first show up on the plastron, not the carapace.
Your tortoise's shell damage is old and will always look like that. There's no need to do anything. It's not harmful nor painful to him, and it's not going to get any worse unless he keeps knocking it on something hard. Is it in an area where he continuously bangs it as he walks along the edge of a stone wall or the stucco house, something like that?
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Yvonne
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