When I had my Ghost shrimp Ash (RIP little buddy) in a 1 gallon tank with some gravel, flower pots, and a Marimo moss ball, the thing thrived.
I have never had any problems with mine. I've had it since around March.
I don't do anything special to my water....
We have well water right under our backyard, we live in the middle of nowhere with no pesticides or anything. The water is very acidic and hard, and maybe a Marimo prefers that water?
I know I can't add salt to the water because it would kill my future shrimp, so scratch that idea.
Anyway, good luck with all your Mosses