what eats flatworms?

[marie]

I had an unidentified damsel that ate them, unfortunately I lost him to a marine velvet out break

[Black Phantom]

The other thing you can do is to vacuum them into a micro filter bag. This works really well if you have a sump or when you do a water change. Just get a smaller diameter hose and suck away.
After a while it seems to collapse the colony and they all vanish