Regarding the use of terrestrial greens, if the practice is really no bueno then why do we see so many public aquariums feeding their herbivores with broccoli, mustard, collard, and other dark leaft greens? They do so because they're inexpensive and they work. All of the hobbyist products in this realm are ridiculously over priced especially the live stuff that you have to have shipped to you. You could buy a lot of Nori from the Japanese market for a fraction of the price of that stuff too.
Inexpensive? You must not live in CA...
That said....
I was reading that they used to use lettuce. That has far less nutrition than the darker greens like spinach. Even with that improvement, though, I believe seaweed contains some things that terrestrial greens just don't.