In the seeds of cereals, the outer covering of endosperm separates the embryo by a protein-rich layer called :

In cereal seeds the embryo is surrounded by the endosperm, which stores nutrients for the developing plant.

The endosperm is covered on its outer side by a single layer of living cells that are rich in proteins and enzymatic activity. This specialized tissue is known as the aleurone layer.

The aleurone layer forms a barrier between the storage‑rich endosperm interior and the embryo, effectively separating the two structures. Because it is composed mainly of protein and is the outermost living tissue of the endosperm, it matches the description given in the question.

Therefore, the protein‑rich outer covering of the endosperm that separates the embryo is called the aleurone layer.