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.