Marimos don’t have many requirements when it comes to housing, but there are a few things to consider. First off don’t keep your Marimo in a spot that gets a lot of sun. They naturally appear on the bottom of lakes, where it’s quite dark, so they don’t react to over-exposure to direct sunlight well.
In the “wild”, Marimo only appears in cooler areas like Iceland and northern Japan. It therefore prefers cooler water – if the container/aquarium it’s in gets a bit hot (25+ degrees C/77 F) during summer, consider moving the Marimo to a slightly cooler place for a few months. It can actually be placed in the refrigerator during the hotter months, but a spot near an aircon is also fine.