moss/marimo balls are Cladophora algae. I don't think they do much for aquariums since they grow extremely slowly (a few millimeters per year), it's more decoration than anything else.
i heard it competes with algae for nutrients. and my shrimp seems to like sitting on it and chowing on what ever gets collected on there. here h[video=youtube;raDecpElpAI]http://www.youtube.com/watch?v=raDecpElpAI[/video]
What kind of moss are you talking about?? Aegagropila linnaei (Marimo Algae) and Taxiphyllum barbieri (Java Moss) are two "moss" balls commonly found in aquatics stores. Both reduce nitrates and suck up excess nutrients.
I just started seeing these fake moss balls this past week. Odd, how hard is it to just have the real ones. I tried some java moss balls before, within a week they were eaten by my red hook mets..
^ X2... never liked them personally... although if your using it as a way to keep the parameters more steady and don't want to use something like duckweed i guess i can understand?