I wish I could offer you some advice here but I have no personal experience with these guys so I dont know if this is normal or not. Do you mean the area towards his tail that looks greyish? If this is what is not normal for him, from the pic it looks like it may have been scraped but if its changing colors with his mood, I dont know. He/she is beautiful though! The best I can offer is to keep a close eye on him and let us know if anything changes such as fuzziness, tufts of hairlike projections, redness/inflammation or any odd behavior. Hopefully, someone with experience with these guys can offer more insight and advice for you!