If you've come across the Leftover Elemental Slime item in Patch 10.0.7 and you are curious about its purpose, combining 50 of them will create a new mount called the Gooey Snailemental.
Leftover Elemental Slime is currently obtainable as a drop from the new primal storm event bosses found in the 10.0.7 version of the Forbidden Reach.
Various elements such as earth and fire can invade the Forbidden Reach, and will appear roughly every two hours.
Primal Storm Invasion Location:
In my opinion, this is a great incentive to encourage more people to participate in the event, even if they don't need the gear or any other drops from the bosses.