I'm not sure if a Lunarre will eat them. What kind of tank is it? A Lunarre will rid your tank of it's clean up crew very quickly if you have them. A Lunarre is also very aggresive, so be carefull what you can put with it.
Other things that eat flatworms (From research, not experience), Arrow Crabs, Blue Velvet Nudibranchs (I just got one of these yesterday for this same reason) and Leopard Wrasses