User blog comment:FairyRaven/Fairy Tail Chapter 398: The Final Duels, Review and Commentary/@comment-7418318-20140901145939/@comment-7418318-20140901193152

If this power really only worked against Demons, I wouldn't be worried either, but Silver demonstrated quite clearly that other beings are just as affected as Demons.

He froze Natsu, a dead Dragon and an entire village of Giants, along with all the inanimate matter in the area.

Of course there is the chance that this was only due to Silver's skill with the ability and Gray's still years away from such proficiency. I really hope this'll be the case.