Clarification is needed it seems-
There are two types-
one is a "ball of moss" which is some type of moss (almost any type) trained to grow over a coconut shell or screen ball.
The other is the Authentic moss ball called a "marimo" ball. The Marimo balls grow in a lake in japan, and people grow them in jars in their windowsill. I would imagine, based on that, marimo balls do not need water current.
Marimo balls grow slowly but can get to a limitless size. (we're talking years and years and years to double in size.
They naturally form a sphere- if you get one, and break it apart, the half will eventually form into spheres (only smaller of course)
I read an article by a pet store owner-
He ordered several baseball-sized marimo balls and had them in a rubbermaid container for quarantine.
He went into another room to answer the phone, came back, and his dog had SHREDDED every one of the balls, with little clumps of moss all over the floor.
He ended up with several dozen tiny marimo balls instead.
Also, it is neat to have them in a current because they will move around, seemingly of their own control.