It looks like a cyst or similar, I've kept ADF's for a long time but not encountered this so not 100% sure.
If it isn't growing or inhibiting the frog I wouldn't worry too much about it. If it does grow larger or the frog stops eating you may need to consider euthanasia.
BTW aquarium salt shouldn't be used with frogs, if you can I'd dilute this out by 10% water change twice daily until you're confident it's gone. I've learned the hard way that keeping fish and frogs together can often cause issues when you need to medicate one or the other as lots of fish meds are toxic to frogs.