And if you go to most stores and buy a pot or pan they advertise as "copper", then you're literally just buying something with copper-colored non-stick coating, and it is not actually made of the metal we commonly call copper. Dumb shit.
And if you really have to do more than the 20 second plain water rinse and rub (never use soap on cast iron), start dry and scrub with equal volumes of salt and oil.
Best I can see when searching, green copper oxide is basically inert biologically. Plenty of complaints about the toxicity of nanoparticles, but that's a whole other game.