The Moon's Tear is found exclusively in Majora's Mask. It is what the name suggests, a shiny rock that falls from the Moon's eye. Link can find it in the gated area of the Astral Observatory, after observing Skull Kid through the telescope. As Deku Link, Link can trade it with the Business Scrub in South Clock Town in exchange for the use of his Deku Flower. This gives him the ability to reach to the entrance of the Clock Tower, where Skull Kid can be found. The Moon's Tear is also mandatory for continuation of the Title Deed trading.
Deku Link picks up a Moon's Tear