To prevent the nut grass from growing back you have to dig deep and remove the grass with the nut attached and not just break off the top of the grass.
So that fruit is not just Mammy, it's Mammy Apple. There are two different types of fruits that I know of, Mammy apple, that one that you have, and another one called Mammy sapote, similar to neseberry but much bigger. It has one or two large seeds that looks similar to neseberry. So that is mammy apple, and there is also mammy sapote.😊