Multiple myeloma (MM) is cancer of the plasma cells, which has numerous effects on the body. It can cause changes in mental status, but this usually not a permanent thing.
The difference with Alzheimer's disease is that it causes gradual decline in memory that does not improve.
So, MM patients can be bad one day, then be completely normal a couple of days later.
Keep in mind that sometimes the treatments for MM can cause memory problems. People with MM are prone to infections, which can also cause memory problems. Lastly, anemia, which is a common finding with people that have MM, can make memory worse.
Because multiple myeloma is a disease that occurs in older adults, this means that Alzheimer's disease can technically occur along with it. Therefore, what I would recommend is that if you are concerned about your loved one's memory, then during the next office visit tell their health care professional about your concerns. They can do a check of their memory and see if there is anything to be concerned about.