One of the most confusing quest in Stardew Valley is the Prismatic Jelly quest, which you can use to search for rainbow jelly. To get Prismatic Slime in Stardew Valley you must first get a Prismatic Jelly Special Order from the wizard.
Once a special order has been received the area can generate slimes 1.2% of the time. Fortunately, Prismatic Slime always drops Prismatic Jelly, so you only need this once.
The best place to grow Prismatic Slime is on the 5th floor of the mine which has a consistent source of mucus with no threat of enemies. You can exit the mine and come back or go to the floors above 5th floor and come back to check the progress and try your luck.
This may take a few days unless you are lucky enough! However, the high spawn rate of slimes on the 5th floor will help you get constant supply of slime.
By bringing prismatic jelly to the Wizard, he will give you 5,000 GPs and Monster Musk. Monster moss increases the enemies spawn rate by 2 times. It is perfect for farming monsters, to collect some rare drops.
Stardew Valley on G2A