Algae (a green or black scum) and moss are usually problems in shaded, over-wet or badly drained and compacted sections of the lawn and garden. If the soil is compacted, spike it with a garden fork to improve drainage, and if there is too much shade, cut back trees or shrubs to let in more sunlight. If the ground is too wet you may have to construct drains to take away the excess water.
Trees which are lacking in vigour and already dying back, often have moss or lichens growing on the bark. It also can occur on healthy trees where the humidity is high. Lichen itself does not actually affect the health of the tree as they are non-parasitic organisms. For lichen in trees, improve air circulation by pruning overcrowded branches and any overhanging growth from neighbouring plants. Improve plant health by watering, mulching and applying a fertiliser.