Most likely more mercury than is found in the plastic alternative. The mercury may be bound somehow to the seaweed in a way that prevents it from being absorbed

Most likely more mercury than is found in the plastic alternative. The mercury may be bound somehow to the seaweed in a way that prevents it from being absorbed when the seaweed is consumed although steeping it in boiling water may leech it out. Ethyl mercury contaminated water is something to be avoided entirely.