That’s just patina, aka what happens when brass oxidizes. they probably should have coated or painted it to prevent that. In any case, while verdigris is indeed toxic, that isn’t an amount I would be worried about. You could simply scrub off the patina and apply a little clear nail polish to fix it.

Is it a good headphone? Yes. At double the price, is it twice as good as the HD6xx? No, honestly I prefer the 6xx. If you’ve tried it and you love the tuning then maybe it’s worth it for you, but no, it’s objectively not a “good deal”, it’s another variation on the 600.