There are a hell of a lot of different complete meal replacements about now. Looking at the recipe of the powder, Mana and Soylent both use isomaltulose and soy protein, but that's about where the similarities end. Mana uses algae, sunflower, flax, oats and coconut, whilst Soylent uses maltodextrin to make canola and algal oil powders, as well as modified food starch. They both, naturally, contain all your daily vitamins and minerals, but the products are pretty different.
EDIT: Mana also has canola oil, I misread.