This is the exterior mantle (named Himo) meaning string of the arc shell. The mantle is sometimes tastier than the clam itself, and the texture is crunchy and concentrated in flavors.
Arc shell is translated as “red clam” or akagai in Japanese for its red flesh. It is high in hemoglobin and therefore gets its red color. When preparing, the clam is shucked and then the valves, and organs are removed and discarded, but the exterior mantle, the membrane that surrounds the clam’s body is saved. After washing, the color of the arc shell becomes pink.