The structure indicated is the omohyoid muscle.
The omohyoid muscle is one of four infrahyoid muscles which lie in the anterior triangle of the neck. The infrahyoid muscles, as the name suggests, attach to the hyoid bone and lie below it. Contraction of the infrahyoid muscles then causes depression of the hyoid bone. The infrahyoid muscles are often referred to as the strap muscles due to their strap-like appearance.
The four infrahyoid muscles are:
The omohyoid muscle sits lateral to the sternohyoid muscle and is comprised of two muscle bellies (superior and inferior) connected by a tendon in the middle.
Origin: superior border of scapula
Insertion: lateral to sternohyoid muscle on body of hyoid bone
Action: depression of hyoid bone
Innervation: anterior rami of C1 to C3 (ansa cervicalis)
Learn more about the muscles of the anterior triangle of the neck in this anatomy tutorial.