Since seaweed is a natural vegetable from the ocean, it's possible that a fish got caught up during harvesting. Some people are super sensitive to shellfish so they have to disclaim that ... to remind people who might have that allergy.
This is a similar concept to the bugs and/or small rodents that get caught up in the harvesting equipment on farms.
It's not anything to worry about unless you have a shellfish allergy. It's just seaweed :)