It’s not Multiple Melanoma, it is Multiple Myeloma.
Melanoma occurs when the pigment-producing cells that give color to the skin become cancerous.
Multiple Myeloma is a cancer of plasma cells.
The plasma cells are a type of white blood cell in the bone marrow. With this condition, a group of plasma cells becomes cancerous and multiplies. The disease can damage the bones, immune system, kidneys, and red blood cell count.

Melanoma is a skin cancer, unrelated to myeloma, a blood cancer. Multiple myeloma means just that, there are multiple myeloma in the system as opposed to a single myeloma.