Although Marimo balls can withstand a range of temperatures and water conditions, they may turn a strange color – this is an indication that something is wrong.
Marimo turning white/lighter means that it is probably receiving too much light. If the Marimo also seems a bit slimy or if its texture seems otherwise unusual, you may be dealing with a hostile type of algae growing around it. These algae choke the slow-growing Marimo, so it’s best to carefully wash them off or remove them with tweezers.
Marimo turning brown may be a sign that it’s time to gently clean it. If this doesn’t fix it, try carefully picking off the brown (dead) parts and adding a bit of salt to the water to stimulate the Marimo’s growth. If the bottom of the Marimo has turned brown because it didn’t receive light for longer periods of time, be sure to start rolling the Marimo around a bit more often to prevent parts of it from dying off again.
Marimo turning black and/or falling apart Marimo are unfortunately known to start decaying from the inside out sometimes, especially when it has been covered by hostile algae for a while or when it is simply too big for clean water to reach the inside. In order for it to become healthy again, the black parts should be removed and the Marimo should be gently re-rolled. It’ll be smaller than it was before, but it now has a good chance of surviving and growing back just fine!