Recent studies (Canada, UK) suggest, that the Japanese seaweed hijiki contains way too much inorganic arsenic to be consumed regularly. In fact, the researchers recommend you abstain completely from said vegetable. The four other major Japanese seaweeds arame, wakame, kombu and nori are all safe for consumption. As for Atlantic seaweeds such as kelp, dulse and bladder wrack I have no information.
Erik F.