There are many things about jellyfish which make them suitable as performers in a music visualization tableau. They are very flexible, so they could imitate the motions of almost any other animal; sometimes, they even have tentacles that look like legs. They come in many forms, and even when they are not differentiated by color, they maintain their identities. The variety is in appearance, too; for a given form of jellyfish, the number of ways it can appear is large. When jellyfish move, the changes in their shapes and their trailing tentacles reinforce the sense of direction of their movement. In fact, the shape of a jellyfish can itself suggest motion (stare at the image to see it move). They are interesting individually, but can also appear effectively in groups. They have a layered appearance, which permits the possibility of different layers doing different things. Jellyfish can be suggestive of other things, such as lightbulbs, mushrooms, and fabric. Because a jellyfish floats in water, it is free to move in any direction; of course, a jellyfish can be beached, too. And, of course, jellyfish are beautiful. They can have a simple beauty, but they can also be complex and dense. Their shapes have many symmetries. And if you want some new ways to vary a jellyfish's appearance, you can get ideas from the forms of other marine animals such as sea anemone, octopus, shrimp, etc.
I am not the first person to feel this way about jellyfish (or, at least, forms that move like them); Oscar Fischinger's early studies featured many such creatures. Like real-world jellyfish, their changes in shape emphasized the direction and speed of their movement.