There are many kinds of slimes. puddings, and gelatins found in dungeons. This one was magically tailored to target the imperially enchanted. Since Royals rarely enter a dungeon, the jelly is often taken to an event where a targeted royal would be at.  
Once the jelly activates and engulfs the target, the jelly quickly bonds and converts the princess into a large glob of jelly. While turning the royal into an easily controlled blob of pearlescent slime, the royal jelly will devour the magical energy before returning to a dormant state.  
The now converted princess is aware but can not move. Instead they are placed in a container for ransom, use in a spell, or added to a collection.
 
Here we have a princess tricked by her student with a royal slime. The student does this to protect the princess and will explain later. For now, its time to scoop up the glowing rainbow shimmering slime and store it someplace safe.