Finely grate 1/2 tsp. zest from the grapefruit; set aside. Slice just enough off the top and bottom of the grapefruit to expose the fruit. Stand the grapefruit on one cut end and slice away all of the peel and white pith. Working over a bowl, cut the segments away from the membranes, letting them fall into the bowl. Then, over another bowl, squeeze the membranes to get any remaining juice. Cut each segment into thirds.
In a small bowl, whisk 2 Tbs. of the reserved grapefruit juice and the zest with the oil, vinegar, rosemary, red pepper flakes, 1/4 tsp. salt, and a pinch of pepper.
In a small bowl, mix the grapefruit segments, hearts of palm, and olives with 2 Tbs. of the dressing.
Toss the arugula and parsley in a large bowl with a generous pinch of salt and 3 Tbs. of the dressing. Divide the arugula among 4 plates, top with the hearts of palm mixture, and drizzle with the remaining dressing, if desired.