How would multicellular organisms evolve in a microgravity ocean?

Life growing from the cracks.

The stuff to build meat will be scarce in these waters. As it is in open ocean, but maybe scarcer than that - at least in open ocean there is the possibility of blown dust, or elements from the sea floor welling up. On this lonely droplet there is none of that.

Perhaps you could have this water in a cloud of space carbon and nitrogen; tholins raining down onto the planet surface. I could imagine they might accumulate on the ice, as might be the case for Saturn's icy moon Enceladus. https://astronomy.com/news/2017/01/gunk-enceladus-surface

https://www.nasa.gov/mission_pages/cassini/media/enceladus_cracks.html

When a crack forms in the ice, that material becomes available to the life beneath. It is an upside-down deep smoker. Instead of rich materials ejected from the deep earth and made available to the sea dwellers, rich materials will fall thru the ice cracks and become available.

I could imagine a tangle of life hanging down from these cracks as they open or close. Primary producers work chemistry on the tholins as they thaw. Heterotrophs feed on the primary producers. Spores or planktonic forms are released into the bottomless ocean, to drift until they encounter another lifegiving rift in the surface.

On one side for a micro organism floating in water, gravity is almost non existent. Being made mostly out water, they simply float in the liquid and are carried around by currents.

On the other hands the lack of gravity will greatly influence the distribution of nutrient: while on Earth any dropping falls to the bottom sooner or later, in microgravity everything, from a rotting corpse to the feces of any other organism would stay in place.

This would mean that transport of nutrients would be greatly limited by only intermolecular diffusion.

As a reference, stagnating waters on Earth are often abiotic or very poorly livable, due to this very issue.