User blog comment:IamJakuhoRaikoben/Chapter 411: Quid Pro Quo, Review./@comment-4933386-20141202035850/@comment-9595671-20141202123535

Only explanation right now is that there's something Mard doesn't know about humans that caused his spell to fail on them.

Actually from what we saw with Silver, I don't think his Ice Devil Slayer powers was a curse, the page doesn't even list is as once. We did consider it was once, but that was before we learned Silver was Gray's father and was just a reanimated corpse.