Hi Gillian:
For the most part, African clawed frogs have a natural immunity to many diseases. Unfortunately, these frogs still get sick. Probably the best advice I can give you is to have your frog checked out by a vet. It could be a bacterial, fungal infection or both. The treatments would be quite different. Watch the frog for the next few days - if the white spot still persists along with unusual behavior, then you need to determine the cause.