I don't think it's pinched mantle since it's been that way for months. Typically clams go downhill when present. If the health of clam starts to deteriorate and no new shell growth is present then I would consider a FW dip.
That does not look like PMD. PMD is progressive. Worst as the day and light goes on. Spread from the disease area to adjacent area. Mantle produce thread of mucus at the injury, infected area.