Moss-shadow (in the game) is one of Bramblestar's children in Mossy's world. She is the sister of Amberclaw and Duskfall. You have to talk to Moonheart when you're a warrior so she'll tell you the story, and then you have to place a specific item on the X that appears at the FireClan border.
There are multiple items that appear after learning about Moss-shadow, such as dirt, a moss ball, a leaf, etc. Think about Moss-shadow's name and the specific item should become clear. Then just wait for Moss-shadow to come.
There was a prophecy at the end of getting your nine lives. If you fail to kill Firestar before the rain comes, FireClan runs BrambleClan out of their territory, and you essentially lose.
