Cave Jellies are strange, glowing creatures found in Phoenotopia, seen only in Mul Cavern. They are small, light blue blobby creatures with tiny feet and bat-like wings, able to fly and occasionally stand on walls or ceilings. They are one of very few non-aggressive enemies in the game and the only type of enemy that can't hurt Gale at all. In fact, they actively try to avoid her if she gets too close to them, making them very hard to kill.
The only way to effectively kill a Cave Jelly is to try and hit one with either the Slingshot or the Javelin. On very rare occasions, a defeated Cave Jelly will drop Jelly Goo, a consumable item that reduces Gale's health to 1 if she eats it but can be given to the Atai Potioner to create the Power Potion.