Are you talking about the wide yellow band around the lower edge? That's new growth. The tortoise has been fed more than he has gotten the past, and has put on a bunch of new growth. Actually we prefer a slower all over growth rather than the expedited growth your tortoise exhibits, but its really ok. What and how much do you feed?
Super healthy looking tortoise. A cup seems like a little too much, unless he really sucks that stuff up. Feed him what he will eat in twenty minutes a day, so that he doesn't get over-fed.