I see that part of his casque is indeed light, but still think it looks like pre-shedding. At times, thermal burns will start out light/white and darken as the damaged tissue dies, but this doesn’t look like a burn and would be an odd place to get one. Fungal infection would be another suspect, but I honestly don’t know enough about them in chameleons (haven’t really seen them occur much on the forum). I’m going to say to just keep a close eye on it and if it changes, post again.