Don't know of a mandarine that eats flatworms. What kind of flatworms do you have? Some wrasse make a meal out of them. The rust brown flatworms make a mess of your tank and release a poison when you smash them. I heard a velvet nudi would eat them. The clear ones that are usually found in refugs people typically recommend to leave them alone. I can't stand the rust ones, but the clear ones don't bother me....can't really see them!!!