Moss balls are the product of a strong oscillating current, hence the spherical shape. The algae that forms marimo balls, Cladophora aegagropila, actually has multiple growth forms- a fuzz on rocks, planktonic fibers, and in aggregate form. The aggregate form is usually in balls, due to centrifugal force of water currents. In order to propagate, it's suggested that you tear the spheres into fibers and roll the initial balls by hand, and then use something like a powerhead in a bucket to cause the fibers to grow together in that shape. If you cut it up, you'll end up with chunks of algae in the same shape they were cut.