Crescent Moons are the game-wide collection players are tasked with completing in The Little Mermaid and the Purple Tide and its accompanying bonus game, Trusty John and King Bluebeard. There are 30 symbols hidden throughout the main game, with one found in each and every location of the game. The remaining 10 are found in the bonus game. Finding all 40 symbols will earn the player an Achievement entitled "Master of the Tides". If the player misses one, they can click back to find it.
Underneath the Hint Button in the game's interface, there is a small crescent moon button. Hovering over this button will tell you how many of the symbols you have found thus far. The moon on this button glows and rotates slightly whenever you're in a location where a crescent moon can still be found. Once the symbol has been located, the moon will turn gray and go still. This mechanism can be used to help the player find any crescent moon symbols they may have missed before completing the game.