In a small saucepan, heat pomegranate juice, sugar, and pomegranate seeds over medium-low heat. Simmer, stirring, until sugar has dissolved and mixture has thickened. Strain and discard solids. Let cool to room temperature.
Place three pieces of watermelon on each plate; drizzle with pomegranate mixture and sprinkle with a pinch of salt. Divide feta evenly between each plate. Place a small scoop of sorbet on each piece of watermelon. Garnish with alfalfa sprouts; serve immediately.