I'm really sorry about your froggy. A fuzzy growth on an African Dwarf Frog is usually a fungus. It's usually caused by fungal spores or fungus in the water or other amphibians or fish or dirty water. This could have happened before you got your froggy. It could have been caused by the place you bought him from and you may not have been able to see it at first. This condition actually is best taken care of by a veterinarian who specializes in amphibians. The veterinarian may administer benzalkonium chloride.
You would still need to kill the fungus in your aquarium. You would remove the animals and treat them separately, remove plants and throw them away, remove fish and keep them away from the frogs for at least 2 months, dry out the aquarium completely- including all the ornaments and the equipment for at least 12 hours.
I'm very sorry for your loss.
I hope the rest of your wet pets are doing fine.