Flatworm exit to remove predatory worm?

[GoFish]

Took the rock out and let it sit in 10 cups of RO and 10 ml of coral dip (precision solutions) for 1 hour. Killed 3 bristle worms, 30 brittle stars and 50 baby snails. No Oneone Fulgida. Then shook the rock around in a bucket of NSWdropping off some more dead snails brittle stars etc, and put it back in the tank
It either didn't die and is still hiding, or died and can't flush it out, or it moved rocks overnight and wasn't in there.

Will have a look for it when the lights go out

I'd leave it out overnight or take more drastic measures like that but I want to put the rock back in and not kill off everything as the tank may not be large enough to handle a mass die off of all bacteria, algaes etc.