Wouldn't seaweed be a net for all of those toxins & pollutants found at sea? Seafood health claims are always focused on omega 3s and such but rarely mention this aspect.
Predator fish like Tuna can become heavier in toxins only because they are the top of a food chain gathering and concentrating said toxins.
Seaweed is just a plant. It grows, then dies if not harvested. It is no higher risk than wheat.