You might be better of to set up a large jar with a soil substrate- (Mix a little soil from outside with some fine gravel and add 1 inch to a jar-, cap with an inch of plain gravel from an established tank)
Add water from an established tank, add the marimo, and place in a sunny window. After the soil starts producing CO2, the marimo should 'come alive' and roll around. They're moved easily by critters in the tank, but photosynthesis also makes them move-
Oxygen builds up in the fibers, and makes them roll.
Ive tried that, it made my marimo float and not really roll. I guess it counts when it floats and sinks, but it grew lopsided anyway so I just tossed it back into the tank. I think it just accelerated the growth, but didn't actually keep it ball shaped...