A clean Marimo is a healthy Marimo! It’s a good idea to pick up your Marimo once in a while (especially when dirt particles have accumulated on it or when it has turned a bit brown or greyish!) and gently wash it by squishing it a few times in a container with some clean water. You can then roll it around in your hands a bit, this helps the Marimo maintain its nice round shape in the wild, it gets this from rolling across the lake floor, which it unfortunately cant really do in a bowl, vase or aquarium, so its a good idea to help it out a bit.