Seaweeds aren’t weeds. They’re vegetables that grow wild in the seas. None are poisonous; all are tasty and nutritious. Many are harvested for special uses, nori for sushi, kelp for noodles, Porphyra for laverbread in Whales. Lyngbya is poisonous, so don’t eat it or any fish that eats it; however, Lyngbya is a cynobacteria, not a seaweed.