This feature of the game is currently Unreleased. There may be changes in the future that will affect the development of certain features of the game.
Quicksilver Slimes are a kind of slime that will be added in the 1.2.0 update, along with with the Nimble Valley as a part of Mochi's Megabucks Update.
Why doesn't the rest of the world know about quicksilver slimes? Why are they ONLY located in the Miles-owned Nimble Valley? I'm sure it's just a coincidence.