You might occasionally hear about toxic algae in the news. Toxic algae will always be in reference to microalgae, or harmful algae blooms (HABs). HABs are responsible for shellfish poisoning and what are known as red tides. HABs can exist in salt or fresh water bodies and are toxic if consumed.
While seaweeds are classified as macroalgae. There are currently no known poisonous or toxic seaweeds in existence. There are a few seaweeds that produce acid (acidweed), but these are no more acidic than your own stomach acid and would not harm you if consumed.
Incredibly there are only 14 reported deaths ever linked to eating seaweed, and the reports state that it’s not the seaweed itself but bacteria that had grown upon the seaweed. We say incredible because there are huge populations (Japan, Korea, China) that consume raw seaweed daily, while in the USA there are 31 reported deaths by E. Coli every year.