A Slime stack is simply an organized stack of three different Slimes: a blue one on the bottom, a red one in the middle, and a green one on top. According to their bestiary entry in IX, a Slime stack is the result of a failed attempt at forming a King slime, as there were not enough Slimes to do so.
Given that they are three Slimes working together, they are fairly formidable when first confronted and especially exceed the sum of their parts. They attack by separating and launching themselves at enemies.
In the 3DS version, Slime stacks appear as special monsters in the A Test of Slime DLC tablet. It is also possible to acquire a slime stick weapon through DLC.
The player may first encounter Slime stacks on Newid Isle, where Alltrades Abbey and Porth Llaffan are, though they will first appear as seemingly normal Slimes. However, these particular Slimes are stronger than those previously faced and can call on other Slimes, forming into a Slime stack when three gather. On Pluvi Isle, the Slime stacks appear as is. It is also possible for a character to dress up like a Slime stack in reverse order (with the blue Slime on top and green at the bottom) by wearing a Slood and Gooey gear together.
Slime stack appears as a C-rank monster of the Slime family as part of the limited Dragon Quest Monsters: Joker event. It can be synthesized in the Synthesis Workshop by combining a Restless Armour Might and a Dragon Slime Might together. The slime stack can participate in Wulfspade's Battle Road as a party member.
The Slime stack, along with its related kin, are possibly a reference to a game-play mechanic in the Slime MoriMori series where it is possible to stack things on a slime, such as objects and monsters, including other Slimes.
It is unknown if the Green slime in the slime stack is intended to be a Lime slime. In some games, such as X, the Green slime is the very same kind as the one ridden by a Slime knight.