Mam, go with your gut feeling. It is there because your logical brain has made a connection. Stop the med and see if the hallucinations go away... if they do then you have pinpointed the cause. If the hallucinations don't go away then you can restart the namenda.
Are there any other symptoms other than the night time hallucinations that cause you to believe that your mom has Alzheimer's? Has she been properly diagnosed? Doctor's, these days, seem to be all too willing to place elderly patients on Aricept and/or Namenda without exploring all the other possibilities first. Sammy and her friend's eventual diagnosis with NPH is an excellent example. There are so many drug interactions and medical conditions that can cause the same symptoms and these conditions may be reversable. It's not that you want to get an ALZ diagnosis, but you want to rule out all the other possibilities.
Welcome to the board and I do hope we hear from you often
Love, deb