Though called moss balls, they are actually algae. And while that sounds like the slime on a rock or aquarium, these balls actually make good houseplants because they don't require much maintenance.
Marimo Moss Ball | The Aquarium Guide
It’s a really neat concept to add algae balls to an aquarium. I had no clue that they can help clean the tank so well. As you said, they evolved from Japan to showing up all around the world. This must show that they are very durable. I would love to see this as a more common agent for the tanks. They seem to let the natural cycle exist within an aquarium.
Marimo Moss balls? - Aquarium Forum
Marimo moss balls do get dirty from time to time. This is normal. A way to remedy a dirty Marimo situation is to clean a Marimo. Yep, you can actually clean a Marimo moss ball. Now, here’s the fun part about cleaning Marimo moss ball. What you want to do is pick up the Marimo out of the aquarium and place the Marimo in a separate clean container of water. And then the fun part. Squeeze! Gently though please. You want to squeeze the Marimo like a sponge, albeit gently because you don’t want to hurt it’s feelings, and release and squeeze and release and squeeze. You can also do this under a running stream of water. Marimo’s collect dirt, debris and algae (not just Marimo algae) not only on the outside service area but within the Marimo moss ball itself. Remember that a Marimo is mostly hollow on the inside.
Moss balls? - Aquarium Advice - Aquarium Forum Community
Marimo moss balls are small balls of green algae that have the velvety texture of moss. They are a very popular choice as aquarium plants due to the numerous benefits that they provide to their aquatic environment. So, what exactly does a marimo moss ball plant do? Let's find out...What better way to combat algae than with algae? Marimo moss balls suck up the same nutrients as undesirable algae, depriving invasive algae of those nutrients. I had brown algae growing in my 10-gallon tank, but since wiping them off the glass and adding a few moss balls, they have yet to grow back. Of course, moss balls won’t make that much of an impact in larger aquariums unless you get more a lot of them. I don’t know about killing off algae, but they can help prevent unwanted algae from growing back.