My Mother's brain cactus(I don't know the real name) she has had it 15 years and it's growing a tan skin. Help

Answer 1

thats the bark forming, its just the bottom of the cactus acting like wood.

Answer 2

If the "skin" is down at the bottom of the plant or the base of the branches (if it has some), it may be normal. Some plants DO develop something that almost looks and feels like bark when they reach "old age". My mom's Christmas cactus is over 50 years old and looks like a small tree with green shoots coming out of it and blooming!