Australian Green Tree frogs are sometimes called Green Tree Frogs. Probably not a mistake. Do you know if yours is the Australian or Indonesian type? It could be a pigment fluke only visible sometimes for all I know.

Keep a close eye on him, see if he spends an unusual amount of time in the water bowl, see if he's eating. If he appears to be sick otherwise, the spot might be related to patchy shedding and a sign of a fungal infection.