Hi there Hyla Sapiens!
For what it's worth, one of my greys had a really similar issue a while back, except her spot was dark, not light (see here:
http://www.frogforum.net/showthread....Grey-Tree-Frog)
In the end the spot went away on its own, and in fact when I looked back at old pictures of her I could see it very gradually coming up, so it couldn't have been a burn like I originally thought. I never did figure out what it actually was.
Also, where did you manage to find captive-bred greys? I am constantly trying to figure out where to get them.