Bring the quinoa, water, and 1 tablespoon of the olive oil to a boil in a saucepan. Reduce heat to low, cover, and simmer 20 minutes. Fluff with a fork and set aside to cool a bit.
While the quinoa is cooking, segment the grapefruits over a bowl, removing all of the white pith and flesh, reserving the grapefruit juice. Place the segments in a large bowl and set the juice aside. Discard the peel and pith, or reserve for another use.
Add the slightly cooled quinoa, beans, walnuts, herbs, and feta to the large bowl with the grapefruit segments. In a jar with tight fitting lid, combine the remaining 2 tablespoon oil, the vinegar, and ¼ cup of the reserved grapefruit juice. Shake shake shake until the oil is emulsified. Pour over the quinoa mixture and stir until everything is well combined. Taste and season well with salt and pepper. Toss again and serve.