I think this method work well for betta owner as betta's poo is large

For plants hobby, one need to be patient and not expect immediate response from the plant especially Marimos, plant unlike animal, which most of the symptoms/processes are very significantly observed in a short time. Either the "greeness" of the Marimo vary according to water parameter (Tank setup) it doesnt matter of dark or light, as long as green, u have nothing to worry.