I tried Salifert Flatworm eXit in a separate container on a dozen worms. 24 hours later at 50x concentration and fewer than half died.
With more reading I found there are many species of flatworms. This one is probably Heterochaerus Australis or similar. Going to try manual removal. I wrote up what I found here:
http://www.theaquariumwiki.com/Flatworm