hmm, that's really odd, since i have used both mineral spirits, non-chor brake cleaner and acetone nail polish remover when i was out of non acetone and have never gotten a cloud like that on any polymer gun, including the 15-22 and some Plum Crazy 5.56 polymer lowers.
i would suggest a sock, over your hand as you would put it on your foot and a quick shot of brake cleaner on the sock, just a shot, then wipe it off. should come off and then look 'dry' but no film. quick shot of rem oil or a small wipe of armor all or pledge will bring the black back.
methinks different batches of polymer are of different QC. the only thing that i have had acetone/mineral spirits/break cleaner attack is rubber, as in stripping a rubber AR grip, kinda melted it, but was using it as a test subject.
should not have clouded at all on that XM15, have the same lower same nail polish, no film...curious....