I'd lean more towards an infection. Can't be 100% sure though. How did it start? Like, was there a small spot and it progressed? Are there other frogs in the tank?
It looks like something similar I had happen several years ago.
Yes another frog in tank.i just bought them 4 days ago. His arm was not like that. No white spots that I noticed.. I put him in the net to get a close up picture and took a qtip to his arm and it looks like the arm is dying off.
Some kind of injury was probably at the root of the problem. You need to treat with a frog safe Antibiotic like tetracycline or doxicycline (API fin and body cure.) Pet store should carry). Follow the dosing instructions and make sure the frog eats.