can anyone explain this.

Well I've had my royal gramma for 8 days noW he eats every time food is put in the tank. But yesturday I noticed a silver spot right where his butt is or where they poop. It's on the bottom of the middle where his colors match. I can't get a picture as he doesn't stay still long enough. And you can barley see it. Just a small silver spot I can't remember if I see it there when I had him or not. Maybe it's normal cause he seems perfectly fine. Swims around. Hides though when he sees my shadow coming though lol. But thought I would ask see if anyone had any ideas