Halimeda is a calcerous algea very few animals will eat it maybe an urchin but I seriously doubt the Red Knob will eat it. I would feed Nori as well as meaty foods such as krill, squid, shrimp etc.
AFA the Linckias nobody really knows what they eat but some suspect the bacterial film that grows naturally on your LR. That's why it's recommended a well established tank with large amounts of mature LR for it to survive long term.