It's either a fungus, mold, or:

Frogs with ranid herpesvirus skin disease develop skin abnormalities which look like grey or white-coloured warty lesions. These lesions are often described as ‘candle wax-like lesions’ as they look as if candle wax has been dripped onto the affected frog’s skin. The disease appears in early spring, usually during the breeding season when frogs are spawning, and then regresses over the late spring and summer. Little is known about this disease but affected frogs do not appear to suffer any ill effects from having temporarily warty skin.

Common toads infected with BfHV1 also tend to develop skin abnormalities during the breeding season, which look like raised black or dark brown patches that can join together to form large lesions. However, unlike ranid herpesvirus skin disease, there is some evidence that common toads with bufonid herpesvirus skin disease may become ill with associated mortality. Further investigation is required to determine the clinical significance of this disease on common toad health.