What are young jellyfish immobile of?

They are called planulae in this stage and are free-swimming larvae. They are covered in cilia, which are tiny hair-like structures that help them move through the water.

However, you might be thinking of a later stage in their development called a polyp. Polyps are sessile, meaning they attach themselves to a surface and remain in one place. They look like tiny, stalk-like creatures with tentacles at the top.

So, it's not that young jellyfish are immobile, it's that a later stage in their life cycle, the polyp, is immobile.