I'm think it's probably just highlighter, marker, or something else that stained her, not mold or fungus. It might have been food too (that color reminds me of my school's chili! Lol XD) I'm leaning towards highlighter if it's neon. Try washing her first and see if that helps. You can try a magic eraser next. Then I would recommend nail polish remover, but be careful as it will remove paint (symbols, blush, eyes). If none of those work, try sunfading.
http://www.mlppreservationproject.com/body.html#SunFading This is a good tutorial :)
I also want to say that you should NEVER use harsh chemicals like bleach on ponies! Bleach will, well, bleach a pony and other things can do bad things such as acne creams which will make white blotches appear that can spread to where you didn't even apply the cream and can spread to other ponies as well! What gave you the idea of using bleach? Just curious.
For the possible age spot on her leg (I use the term age spots because cancer can offend some people) there isn't much you can do. Sunfading doesn't really work. You could paint over it if you want.