We'd read about the Earth Cafe before leaving for Marrakesh so it was on our list of places to go however it took us a long time to find it! The easiest way to find it (in my opinion) is to look for El Waha Cafe on the South border of the Square and take the alleyway to the left of that. Keep going up that alley, further than you think and it is signposted up high.
Lovely, bright place but I do agree that a little sparse - some pictures/decorations would make it a bit nicer. Staff very friendly and the food was lovely. We visited twice and had a different meal each time and different juices which were all lovely. The only complaint that I would have is that the food isn't that hot when it is served and as the burgers contained rice I was a little concerned (this was unfounded though as we were fine!).