There are numerous chemical controls and mosskillers that can be used to kill the moss. However, moss is a symptom of poor growing conditions and/or poor lawn maintenance, and unless the underlying problems are corrected, it will just keep coming back. Poor growing conditions include very moist or wet conditions, shade, very acidic or very alkaline soil pH and soil compaction (especially on clay soils). Poor maintenance is usually down to irregular and/or incorrect mowing and a lack of feeding.
Improve the growing conditions for the grass – reducing shade, raking out thatch (dead grass and other debris) and reducing soil compaction by aerating with a hollow-tine aerator and applying a lawn topdressing. Feed the lawn throughout the year to keep the grass growing strongly and help out-compete the moss. And, most importantly, mow regularly to keep the grass at a height of around 4cm (1½in) high all year round, 5cm (2in) or even more in deep shade.
You could try to physically remove it, but this should be avoided, as it would probably do more damage to the bark and branches.
Moss can be controlled to some extent by improving air circulation around and through the plant – prune out any overcrowded branches – especially old branches –and remove vegetation growing around the base of the plant.
Improve the overall growing conditions for the plant to improve its strength. Look at the condition of the soil and whether it becomes too dry in summer or overly wet or waterlogged. Ensuring conditions for good root growth will help to increase the plant’s vigour. Water if and when necessary, mulch the soil and feed annually in spring with a granular fertiliser. Applying a foliar feed, especially of a seaweed-based tonic, may help to improve strength and vigour.
On hard surfaces
Mosses can usually be controlled by physical removal, such as with a scrubbing brush, or use a pressure washer – although this might just spread them further!
On the soil
Moss growing on the soil is a sign that the soil is compact, airless, may contain too little organic matter and remains moist for much of the year. It is usually only a problem on heavy clay soils.
The moss can be scrapped off, but it will return unless something is done to improve the compaction and drainage of the soil. Dig over the soil and dig in bulky organic matter and even sharp sand and/or horticultural grit.