He is half mutant himself as well, i was just looking him up and his supposedly half amenthian mutant and half daemon so he would def have some inside info. And it also seems that yes, his powers do only work on mutants. It also now thinking about it, makes no sense that he would have only changed half of Storm if he could have just turned her completely into something that could not fight. Now i wonder if storm was testing herself more than we know.