While real sea-grown algae might be light what will happen is the same as with modern animal and plant farming: animals are fed corn which makes their meat sub-par nutrientwise and most of the soil for plants is depleted.

So if we now grow algae in "reactors" their nutrient profile will be artificial and out of line of their natural counterparts, thus making them no better than any other modern highly processed food, so we might as well stick to rice and potatoes in that case...