I have a bunch of marimo balls (moss balls - but they are actually algae not moss) in my tanks - I really like them for some reason.
Where they grow in the wild, they form balls due to the waves rolling them around. If you don't have a current in your tank that is strong enough to move them around a bit, you need to take them out every once in awhile and roll them around in your hands - or they can eventually loose their ball shape.
They are able to float or sink in response to light levels. When they get a lot of light, the process of photosynthesis causes air bubbles to form within the ball, which then causes them to float closer to the light source.
Some people like to rip apart the balls - if you tie the pieces to drift wood or some other type of anchor they will eventually grow to cover it. They can grow to make a really nice bright green algae carpet.
They are suppose to be good for fish tanks, because they use up a lot of nutrients and can keep levels of other types of algae down by using up all the resources. Plus, where they are found naturally in Japan, they are considered to be good luck!
Um unless you have some other kind of moss and not a marimo ball...then just ignore everything I said!