You have to hatch Milcery from the eggs
You can either get Milcery eggs from the lab or breed an already
hatched Milcery with a Pokemon that has Fairy Egg Group or Ditto.
However, the chance for breeding would be 50:50 if you breed with
mons other than Ditto
For Tall Grass to work, you need to have EggDex of Milcery though
:/