Hi Jeppie,
It looks like an injury of some sort, though its hard to see well. Is he still eating?
Please answer these questions so we can help...
http://www.frogforum.net/african-bul...enclosure.html
For now, make him a hospital tank with paper towels as substrate to keep the area as clean as possible. Very gently trickle some fresh dechlorinated water or spring water over the area to clean it. Do not touch or rub it, just let it rinse off. Allow the area to air dry as much as it will. Use a Qtip and apply a little blob of Original Neosporin (NOT the pain reliever type -it is toxic to frogs) to the red area. Try to cover the whole area. By using 2 Qtips and squishing the neo into a nice little blob on the end of it and then rolling the Qtip around the toe area, it should glob on. You don't need a lot, just enough to cover the red areas.
I would contact a vet. If there are no exotic vets in your area there is a vet that often helps via phone and email.
If you can please copy and paste the link above and answer the questions, we will be able to help more. A clearer photo will also be greatly helpful.