Hard to tell from the pics. Since it happened so quickly my first guess is something beat him up. What else do you have in with him?

Disease wise the only possibilities that come to mind are bacterial or uronema that could look similar and happen so fast.
For bacterial, you would treat with antibiotics. I'd probably go with Furan II. Course you'd have to remove him but getting a trigger out of a reef tank is easier said than done. For uronema, well game over when it's that advanced.