The structure indicated is the semimembranosus muscle.
The semimembranosus muscle is one of three muscles in the posterior compartment of the thigh. The other muscles are the semitendinosus, and the biceps femoris.
The muscles in the posterior compartment are often referred to as the “hamstrings” muscles. Collectively, these muscles are responsible for extending the hip joint, and flexing the knee joint.
The biceps femoris lies laterally, and the semitendinosus and semimembranosus lie medially.
The semitendinosus gets its name from its structure which is comprised of a muscle belly which tapers into a long tendon that extends to insert onto the medial tibia. The semitendinosus sits on top of the semimembranosus. An easy way to remember this relationship is semitendinosus top.
You can learn more about the muscles of the posterior compartment in this video.