So, the moss ball is only one form of the algae, it has two other forms one of which is a "carpet form." They live in streams where they're rolled around which helps them keep their shape but in aquaria they don't have this so you have to do maintenance on them by rolling them between your palms once a week like you would play dough.
Marimo can live for a hundred years and replicates by budding little balls off of them that detach and form another moss ball. You can also force it to split yourself, the sides will be brown from where you forced the division by within a few days the inactive chlorophyll will wake up, leaving you with two green balls.
Since they use the same resources as algae pet stores recommend them as a natural way to HELP combat it, but they're obviously not really a solution. A lot of fish don't find them appetizing either so it's safe from most plant-eaters.
Debris also tends to settle on them so it's nice when during your weekly rolling of the Marimo to run it under cool water and squeeze any debris from it. This will cause your Marimo to float for a while until it reabsorbs water and sinks back to the bottom.
Goldfish love eating Marimo, so don't put them with goldfish.