It is a property of language of Turing machines (recursively enumerable languages),
Using Rice's theorem, there exists Turing Machine $T_{yes}$ for $L=\{a, aab\}$ and Turing machine $T_{no}$ for $L=(a+b)^*$ and also $L(T_{yes}) \subset L(T_{no})$, making the property non-trivial as well as non-monotonic.
Hence, the above language is not recursively enumerable as per Rice's theorem part 2.
https://gatecse.in/rices-theorem/