Many types of mosses could be growing on your bark. The ball-like bromeliads that you see attaching itself to the bark and branches of your trees is often called bark moss, but more appropriately, it is called ball moss (Tillandsia recurvata) and like Spanish moss, it is not a fungicide. On the other hand, the green moss that grows on bark is a fungicide and it uses the tree's nutrients and water, causing possible harm to the tree. Whatever the type of moss you have, you can use baking soda to kill it without worrying about harming your tree in the process.
Soak the moss with water about 2 to 3 hours before you use the baking soda. This will help the moss absorb the baking soda.
Mix ½ pound of baking soda to every gallon of water. You can also add a surfactant, such as a horticulture oil like SunSpray UFP. Surfactants will help the mixture adhere to the moss's surface. Follow the dosing instructions on the label since each type of surfactant is different.
Spray the moss well. It should eventually die, fall and/or stop growing. If it continues to get larger, it is not dead, so reapply the treatment.