growing something ON it? a pic would help. is the water in tank clean? anything new you are feeding? try not to feed live tubifex/worms as they water from which they come can have many infestations.
I've heard of dead fish having their skin become more transparent and showing their intestines and what not (which i think are green) but never a live one. :O I don't have any goldfish experienc, so I'll leave it to the experts. Just giving my 2 cents.
ya i think if you are overfeeding the insides might be getting pushed towards the skin or something, but this is weird. if u can post even a crappy phone-cam pic, try to, this is interesting.
could it be a reflection on his scales? do you have green plants or green rock/gravel in your tank? green color may just be getting reflected off his underside.
pics we need pics!