The valve you refer to is a ball valve and the green color is probably normal for bronze materials. Do you have a threaded thru hull then a ball valve screwed to that? Or it could be a real seacock but O'Day didn't seem to use that option. I doubt you have a dissimilar metal problem most likely just normal corrosion of bronze, clean it up and see how it is getting wet. If the through hull is leaking you need to fix it. If the valve is leaking you could be leaking holding tank stuff into your boat, nasty.