What's wrong with my clownfish?

[Zoidberg]

For two weeks my male black ocellaris has had a small, dark growth on it's bottom lip.

- He's very skinny now but eats every time I feed the tank.
- He has been resting on the sand a lot.
- Once in a while he'll trash around for a few seconds.
- Doesn't seem have any trouble swimming or breathing.
- No other fish has these problems
- He has never been harassed / in a fight

What might cause that growth on his mouth?