That does not look like a "spot" to me. That appears to be more like a bacterial infection that hasn't gone away. How long has it been there? How old is this frog?
It would be very helpful if you could share a picture of your vivarium because then we could see what the conditions are like.
If you haven't already, I would consider quarantining it from your tank until you figure this out.
Are you able to provide any other views of the lesion?
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