Marimo (毬藻) are balls of algae most known for being native to Lake Akan in Japan, colonies are also found in Iceland, Scotland, and Estonia.
They like cool filtered water and indirect sunlight or bright artificial light, and need to be pushed around gently from time to time to mimic the currents in their natural habitat, and to give them light on all sides, keeping them green and happy. You also need to change the water about once a week, and occasionally clean out the container.
You can also make them extra happy by putting them under gently running water and rolling them around to clean off any debris that might catch onto their velvety surface (dust, cat hair…) and can gently squeeze them, run water over them, and then repeat a time or two more to get cool, fresh water to their centers. Make sure to roll them around in the palm of your hand to help them maintain their shape. But gently, of course!
If their water gets a little too warm, just move them further away from their light source and put in a small ice cube, or if it’s really warm where you live, you can put it in the refrigerator for a few hours! But note that they won’t survive freezing temperatures. If the water gets really warm before you do any of this, change it out completely.
Very popular in Japan and other places for being a low maintenance plant, it is said that those who own them will have good luck!
my own experience