One side effect of using Flourish Excel that hobbyists have reported is that certain types of algae growth tend to lessen when Excel is dosed in an aquarium. Black beard algae seems to be particularly affected. Marimo moss balls aren't actually moss, but are actually a type of algae. Because of this, many people have been concerned about using Excel with marimo balls, however, the majority of the feedback we have gotten is that Excel has no negative effect on marimo balls when used according to directions. Here is a link to a forum post where aquarists are discussing their experiences with Excel and marimo balls.
Because some people have observed reduced algae when using Excel, some people try to use it to treat unwanted algae. We do not produce or market it as an algicide and using it as spot treatment where it is overdosed or applied directly to a plant can cause damage.
With marimo balls or any sensitive plant you can begin with a small dose and increase slowly over time after observing for any negative reaction. If the moss balls are the only plants or the tank is sparsely populated, reduce the dose accordingly as they are low-light, slow growing plants and will not use as much carbon as other faster growing plants.