Polyclad Leopard Flatworm
Pericelis byerleyana which gets up to ~2.75"/7cm and (probably) eats snails. You might be able to get away with offering frozen snail meat (surprisingly readily available online) if you really want to keep it without offering it live snails - no guarantees it would take frozen meat or derive the nutrition it needs from any of the frozen snails available on the market though.
Edit: Knew I remembered something about these guys feeding on things beside snails - Exnisstech had one (closely related
Pericelis tectivorum) that ate clam meat, which may be a good option too:
https://www.reef2reef.com/threads/hungry-flat-worm.1059959/#post-12622181