There be a killer in these here waters

[Gizmo]

So I lost a black perc about a week ago. Never found the carcass. I though it was one of those things where you buy a new fish, sometimes they just don't make it. But yesterday I lost my royal gramma. That carcass I found, half there with barely any color. And this morning my regal has a nip out of its tail. I'm starting to think I have a rouge mantis, but when I feed, nothing comes flying out of the rocks for food like the last one I had 6 years ago (different tank) Is there any way of finding one if it exists? - thx