Just wipe it off, there will be no harm in it.
If it does not grow back, good for us!
It most likely was a tiny speck of skin that did not come off during the shed you cannot see,
or a slight cut or abrasion your frog got.
Seen it many many times.

As for small tanks, and cycling, it's just a no go.
Seen them crash too many times, and it's a real drag.

I just cannot agree with you about the ammonia.
I know that the API tests read the NH3 and NH4,
but in a tank that size, I would jut do 100% changes no matter what.
You will not stress the frog.
Frogs are different than fish, I've commonly done 100% changes with my frogs,
and they are never stressed, if anything the get excited and want to breed.

If you move up to a 5G, the cycle will hold easily,
and you can add the gravel and filter from this tank, because the beneficial bacteria have been
building up in the filter for as long as you've been running it.
It will help get the 5G cycled more quickly. You won't need all those products.
It's just that the volume of water in a 3G makes it much harder for the bacteria to colonize enough to keep it all stable.