Thanks for writing to us.
It would be very difficult to diagnose the underlying cause for such pigmentation without examining the lesion.
Momate s contains a mid potent steroid mometasone
with salicylic acid
. Your mother has been using the cream for a long period of time. When used for long steroid can cause thinning of skin in the area . Salicylic acid can irritate the skin leading to burning , itching or stinging sensation over face. So she should stop momate s cream and may apply a mild moisturiser to soothe the area.
I would advise that you meet your consulting dermatologist
for further follow up and advise.