Without a picture, it will be hard to diagnose. Does the white spot look like cotton tufts? If so, it is likely a fungal infection. There are many anti-fungal remedies used in treating tropical fish, like Mar-Oxy. Treat your frog according to label instructions.

Saprolegnia infections are a result of poor water quality and large amounts of waste in the water. It can also be transmitted between fish and frog. Make sure you do weekly water changes and siphon out any debris in the water.