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.
A prototype version of the Quicksilver Slime was added into the game's files in the Update 1.1.0. It looks like a Fire Slime without the flames and its plorts look like tabby plorts. It can't eat anything and it moves around faster than the other slimes.
Recently in the Update 1.1.2, the slime and its plort have been made unvaccable.