Never clean plastic with chemicals. Try warm water with a small amount of soap, and use a lightly damp cloth using that mixture to wipe down the joycon. I also have sweaty hands and haven't been able to cause permanent stains.
If it isn't just stained, you're pretty much sol. From what i hear, the finish on the switch and joycons aren't all that great in the first place. With skins ruining the finish etc, I have a bad feeling that your issue is permanent. There is a cleaning product here in the states called goo gone. It is citrus based and meant to destroy sticky residue and the like.
WD40 works great on acrylic which is probably way sketchier than what the switch is made of. Try that on an inconspicuous spot and see. If it doesn't work right away let it sit for a while like over night.
Although you can most likely polish the screen if it's scratched, I doubt you can polish the matte finish on the joycons, they seem to have been made to be disposable. I suggest you enjoy playing your switch until you wear out the joycons then buy another pair, or you could buy a pair of blue ones right now they look cool or get some joycondoms.
pix is crappy cant see anything.
The top mark looks like a white scratch.
Normally i clean stuff with just a baby wipe or mircofiber and warm water. Just wipe it down.
IF its not coming off then its something else and to strong of a cleaner will peel or dull the paint.