Flatworm exit to remove predatory worm?

[GoFish]

I have a predator worm in a rock (oenone fulgida) just trying to get rid of him and perhaps it's babies. I tried taking the rock out and pouring club soda all over the rock as per a LFS staff suggestion, nadda. Then tried electrocuting the rock with a 12 v car charger at 15 amps for 5 minutes in a bucket of water, nada. I know it's in there.

Now. Just started reading about ridding flatworms and it seems like some people are saying that other worms and brittle stars died using the product? If so perfect, I can just take the rock out and place it in a bucket and treat that rock?