From Deanos link: Flatworm babies: Marine flatworms are hermaphrodites, that is, each flatworm has both male and female reproductive organs. When two flatworms meet, they exchange sperm. Some species simply insert their needle-like penis anywhere in the body of the partner. This is not surprisingly called 'hypodermic impregnation'. In yet other species, each flatworm tries to impregnate the other without itself being impregnated, as it involves more energy to produce eggs. This results in a sort of 'penis-fencing' when the two flatworms meet! :Starwars:
That flat worm eats snails and clams. They have poison that paralyzes the snails. I had trouble catching one because they are very slippery and faster than you think, once they fit in a crevice in your LR, it is hard to get out. I caught mine with my trusty turkey baster, sucked it right up.
It is a very cool critter, but I think you better take it out of your tank.