I know that aquatic frogs are quite prone to fungus. Yours is probably suffering from this--does this whiteness protrude as small filaments from the leg, or is the leg simply discolored? If the former (filaments) then you need to buy Maracyn for your frog and dose your tank.
Maracyn kills off fungus effectively. Somebody PMed me about a similar case with their frog and I recommended Maracyn: it worked.
If the leg is discolored, it may be gangrenous... I don't think that this is the result of an uncycled tank, though. If it were a bodily response to high ammonia, the entire body would be turning white.