I would dip it like it was a piece of coral, were I to have the same concerns. At the very least a fresh water dip in cold RODI. Flat worms typically do not enjoy fresh water.

You will probably experience a mini cycle as well.

If it's truly a concern I would put the rock on your porch for a few weeks. Spray it with water periodically to get rid of the die off material. I recently put some rock out for weeks to get rid of the texas trash. I was horrified to see the polyps starting to inflate after reintroducing it to the tank. May have been waterlogged dead material, but taking no chances I peeled off the remnants.

Doing this will stay the bacterial filtration for a bit, but it would also kill the possibilities of unwanted pests.