My biggest suspicion would be that this guy is tail-biting. Usually seen in veiltails since they grow the longest fins...but any long-finned bettas are susceptible. Not really clear what causes this, there seems to be all sorts of theories but none of which is concrete, IMO.
BTW, that is probably one of the best looking double-tail bettas I have ever seen. And I'm not even a big fan of double-tail bettas.
Pearling.....it's way overrated.