A: There are two occurrences of green growths on trees, none of which are harmful. More than likely it is a moss, which is a simple plant. The other possibility could be a lichen.
These growths, although not aesthetically appealing to some, do not need to be removed.
Growth and proliferation of mosses and lichens are an occurrence of a suitable growing environment such as shaded moist environments. Very sunny areas that have excellent air circulation usually do not have lichens or moss in any great extent. Deeply fissured barked trees such as ash and chestnut oak usually have an abundance of moss growing on them.