Is he a new addition? When I first got mine he went through a phase where he wasn't eating well and periodically went pale and developed strange bumps. He'd look like nothing on Earth - skinny, bumpy, lacking colour - one day, and fine the next. Once he started eating regularly and settled in, his colour came back and the bumps disappeared for good.
If he's not new, and it's happening mid-day, it could be a sign of stress. If you're looking at him in the late evening or early morning it could be normal - lots of fish become more pale during the night.