It could be crazy built up shed that has had time to form lymph fluid and then crust over like a scab/ scar tissue

Also it could be fungal, in which case you would need a fungal ointment and I couldnt tell u which would be safe for an iguana.

Best to take him to a vet, but you can also try warm water soaks, clean it with betadine and if it is build up shed, mineral oil may help relieve it.