Was he fighting with other fish? It is a probability that he wa fighting and got nipped. The scale pop off should heal, but if the fighting continues it could be bad like death bad lol....
Just keep an eye on it to make sure it doesn't start getting infected or that more scales start popping out. Otherwise it could have just gotten caught on something, been caused by a fight, or a number of other things. You could try to add some melafix (I usually do a half dose because my fish start breathing heavy with a full dose) which has always helped wounds heal faster IME.