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.
