Were the planaria observed in the process of doing the damage and not just at the site of existing damage? This looks like the kind of damage commonly done by fish and even other snails. Unless you have observed the Planaria in the process of causing the damage, it is improbable that they had any role in large-scale tissue damage. Once the damage has alraedy occured, Planaria will then be attraced to those areas because there is dead/damaged tissue present. Planaria are also attracted to infected areas on snails for the same reasons, but are not the source of the damage in those cases.
In some dutch discussion boards a few other people say that planaria does nothing and other people say that the planaria hunt their snails
This idea that Planaria can attack and harm larger animals is often due to incorrect assumptions and/or misidentification of the worms. I have yet to see someone present any form of proof of such a thing being caused true Planaria, while there are many stores of people attributing damage to animals that are unrealistic and based on incorrect assumptions.