You can use moss for guppies, especially the java moss. Make the moss sterile by having a fertilizer regimen and improving the water quality. Mosses produce life-giving oxygen and release it into the aquarium water for the benefit of aquarium life.
Just give them normal food and some plastic plants to hide in. The moss from a creek can't be sterilized to be safe for domestic fish and can bring harmful diseases, bacteria, and fungus into your tank.