Without the help of these algae, the coral cannot survive. Without the algae, the coral cannot display the beautiful colors that we see in the reefs. If the algae leave the coral, it will white in color. If this continues for a long time, the coral will eventually die.
On the left is a healthy stony coral. On the right is a stony coral that has lost its zooxanthellae cells and has taken on a white bleached appearance.