Since it is the same area, ask her what she sees. Maybe she is seeing something there and is trying to remove it.
If so, perhaps an attractive hat she likes can cover the area and eliminate the vision or distract her from it.
Beyond that, I have no idea what to do. She might have vitamin/nutrient deficiencies and is unaware. Or, perhaps, a drug interaction is creating the condition. It's hard to know without the full details. A competent doctor should be able to help.
What stage of AD is your mom?
Pat