Working over a large bowl, carefully carve out sections of grapefruit and orange from membranes using a paring knife, letting sections fall into bowl and reserving membranes. Transfer juices to a small non-reactive bowl; squeeze membranes into bowl. Discard membranes.
Add lime juice, cilantro, red pepper flakes, and salt to the small bowl with the juices; stir to combine. Add jicama, apple, and cucumber to the large bowl with the fruit. Pour juice mixture over fruit mixture. Gently toss to coat. Let salad stand for 10 minutes before serving.