The Cross is an item found The Adventure of Link.
The Cross is located in the Three-Eye Rock Palace. In order to obtain it, Link must once again defeat Rebonack, as he did at the end of the Maze Island Palace.
With the Cross in hand, Link is able to see blue and purple Moas that are normally invisible. While technically optional, the passage to the Great Palace is markedly more difficult without it.
Link acquires the Cross after defeating a Blue Iron Knuckle in the Three-Eye Rock Palace