Seaweed can provide some vitamins and minerals that are beneficial for cats in small amounts.
Risks
Cats cannot digest plant material well enough to extract all the nutrients.
Overconsumption can lead to digestive upset or other health issues due to its high iodine content.
Suggestions
If you want to give your cat seaweed, make sure it is plain and given in small amounts.
Monitor your cat for any adverse reactions.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.