My grandmother told me that she thinks grandpa should see an alienist. Does she think he's from another planet or what?

Perhaps. More likely, she's using a word that once commonly referred to the psychiatry profession. Alienists were the doctors who dealt with issues of the mind. In newer times, the alienist title associated with a trained expert who might deliver testimony in a court trial, helping to determine whether someone was mentally competent to take the stand.

From Joseph Conrad's Heart of Darkness: "Are you an alienist?" I interrupted. "Every doctor should be - a little . . ."