I have a strong suspicion that a spinach salad, or some spinach in a smoothie or what have you, would be just as good. Not as convenient as tablets, but if you're eating a wholefoods diet it's not that hard to eat green leafy vegetables several times a week, and you'd probably get much more out of a single serving. As Tommy said, those supplements are processed and will presumably have lost nutrients as a result. That plus the fact that "superfood" or not, you're eating a tiny quantity when you have a supplement, as opposed to the far more substantial quantity you eat of the fresh vegetable - which also includes fibre and water and other important things.
Are seaweeds meant to be superfoods? I try to remember to put a bit of wakame in when I make miso soup, which I generally have a few times a week. The quantity is much smaller than the amount I'd be eating of spinach or pak choi or what have you, though, but certainly as far as flavour is concerned, seaweed is very concentrated stuff.