mantis shrimp are filter feeders just like you're a mantis shrimp... lol
Ideally your mantis will take frozen mysids, but if you need something alive the variety of mantis would determine what to feed them... spearing species will take small fish, while smashers do better taking down small shrimp, snails, or crabs, I wouldn't offer them prey larger than half of their own body size
they will eat mysid shrimp, krill, and pieces of squid just make sure to use a feeding stick or tongs so you can avoid being injured by the mantis's powerful mouth. They don't call them thumb splitters for no reason
their mouthparts aren't what you need to watch out for, they're nothing special, it's the foremost legs on all Stomatopods that are modified into some sort of offensive weapon, some have a modified club which they use to break open shelled prey while others have barbed spearlike appendages used to pierce soft prey and rend flesh while they prey tries to escape, both types of weapons can be used to spectacular effect on human hands as well as prey items...