This trial is testing whether taking beta-glucan by mouth can help boost the immune system in patients with advanced melanoma who are already receiving Pembrolizumab. The goal is to see if this combination can make the immune system more effective at preventing cancer from coming back. Researchers will look at how well the immune system works and track patient outcomes like cancer recurrence and survival. Recent studies showed promising results about the combination of β-glucans and immune checkpoint inhibitors for patients affected by different solid tumors.