Fire Belly Toad has a White/Grey Spot on Head
Fire-bellied toads (Bombinas) can suffer from a large array of aquarium problems caused by fungal, viral and bacterial infections. If I were to guess I would say it is fungal or bacterial. It looks like a secondary infection following some sort of trauma.
I cannot diagnose your specific situation, however if you have a hand lens, take a close look at the patch and see if it is fuzzy. That would indicate a fungal disease rather than a bacterial disease.
Good husbandry and sanitary conditions are a must. A mesh top to the aquarium will permit good ventilation. If you are using a water filtration system, the water should be completely replaced every four weeks because toads expel large amounts of waste and the entire aquarium should be thoroughly cleaned every 45 days or so. If you are not using a filtration system, approximately one third of the total water volume should be replaced once or twice a week. Always use non-chlorinated water.
Maintain temperature levels of the vivarium in the 65-82 degrees F range – a temperature gradient is best. Finally, you should not handle your Bombina with bare hands because the residue oil on your skin can harm them, so use latex gloves whenever the situation calls for handling your toad during cleanups.
The best thing to do would be to take it to an amphibian veterinarian. If you do not already have one, here is a website to search for one http://arav.org
Barring that you might try Silvadene (aka Thermazine cream) It's broad spectrum effective against a range of organisms. 1 to 2 applications daily for 7 days or until symptoms begin to resolve. Move toad temporarily to a tank with water less than half its height, so the stuff isn't readily washed off.
Wet paper towel is a fine temporary substrate