Well it's no doubt that the pixie you picked up is clearly a giant by the overall look. The yellow spot I have never seen before, looks to be some color pigmentation like you said. I don't know if this was caused from UV radiation, poor conditions, or a genetic trait which would be quite rare. Yeah he doesn't look to great right now; I would get some calcium w/ d3 in him right away dusted on his food. Additionally the multivitamin to get him healthy again. The 6 years old sounds a little far fetched, but if that is true, then that's very sad and pathetic.

I would hold off on picking him up right now even though his temperament is good because his bones look to be fragile from lack of calcium (Metabolic Bone Disease). Best thing to do is get him back on a regime of a steady diet and supplementation. The water dish he's in is just water deposits left on the side - just wash it out and treat the water again. Thanks Rob for picking him up - respect to you. Not many people would want to recover a frog in that state I would assume. The slime or gooey substance on him appears to be shed skin; I think you're correct.

Grats on all the other pets you picked up and now you're a pacman owner! Haha sweet.