The texture doesn't look right for a Polycladid, though it's possible it may be one. All Polycladids are carnivorous and are specialized predators, depending on the species they'll prey on dead organisms, sea squirts, worms, crustaceans, corals, snails while others are canabalistic.
Whether it is Polyclad or a Nudibranch unfortunately without their prefered prey they will starve to death.
Regards,
David Mohr