The Moss Key Pin was a pin in Club Penguin. It was hidden in the Underwater room, and only members could obtain it. This pin was required in order to enter the Underwater Room from the Hidden Lake. In order to access the Underwater Room, the player had to follow the Giant Squid in the first underwater level of Puffle Rescue. The squid would lead the player to a hidden entrance, and from there, they could obtain the Moss Key Pin.
The only way to find this pin was to be in the Underwater. However, it was impossible to get into this room without the pin unless the penguin played the game Puffle Rescue.
In the game, the penguin had to play the members-only Underwater Level of Puffle Rescue. After saving the black puffle, the Giant Squid would appear and swim out to sea, leaving a trail of bubbles behind it. The penguin had to use those bubbles to chase the giant squid to a coral reef far out to sea, where there would be a staircase which led into the Underwater room, where the Penguin could get the Moss Key Pin.