Bring to Gorman to get an Empty Bottle
The Mystery Milk is a special type of Milk found exclusively within Majora's Mask 3D, required for the Fraternal Milk side quest.
From 6 a.m to 12 p.m on the Second Day, Gorman is found sleeping in the Stock Pot Inn. When Link speaks to Gorman while wearing the Troupe Leader's Mask, he asks Link to bring him special milk made by the Gorman Bros that instantly heals an upset stomach.
When Link then speaks to the Gorman Bros at the Gorman Track from 12 p.m to 6 p.m while wearing the Troupe Leader's Mask, the Gorman Bros give Link a Bottle of Mystery Milk, but it spoils after just 2 minutes, so Link has to deliver it quickly before it turns into Moldy Milk.
When Link delivers it from 12 p.m to 6 p.m to Gorman, Gorman claims it "brought him back to life", and lets Link keep the Empty Bottle in return.