I agree with the dried seaweed except that it is very hard to find real good quality organic seaweed in the oriental markets, the cheap stuff they sell is better than nori but still not good enough for tanks that you want to control the input of nutrients, like everything you get what you pay for.
IMO is better to not give them neither, let them do what they do best in nature, scrape the algae off the rock, it is much easier to eat the seaweed once they get use to it than having to scrape it of the rocks.
Instead complement your food with a spirulina base food or add spiraling to your own food, you can get it at any vitaminshop.