Decorative algae, y’all.
Marimo moss balls, Aegagropila linnaei, are a species of algae typically found growing in wild colonies in Japan, Iceland, Scotland, and Estonia.
But now, you can find them at some plant nurseries with aquatic plants, or your local PetSmart (for quality moss balls, I would refer you to local stores).
Marimo are a great plant for beginners, requiring little care. Change the water every ten days, or when you notice floating particles in the water. As the water becomes darker, Marimo struggle to photosynthesize. Ensure their longevity by keeping them in clear water in a shady, temperature controlled spot.