Moss is a tiny plant that spreads readily in moist, shady conditions. If you don't move your lawn furniture often, particularly if the furniture is wood, moss may begin to grow up the furniture legs and even further up, onto the seat and arm rests. It will continue to grow as long as it isn't removed or disturbed--an entire piece of furniture could end up covered in moss. Removing moss from lawn furniture is not difficult, but does require some scrubbing, as moss tends to cling.
Spray your lawn furniture with a garden hose set on high pressure. You may be able to get at least some of the moss off this way.
Mix a mild, biodegradable detergent (so you don't hurt your lawn or nearby plants) with warm water in a bucket. Use 1 tbsp. of detergent per gallon of water. If instead of a common household detergent, you use a soap-based garden product created specifically for moss and mildew removal, follow the mixing instructions on the label.
Completely saturate a large sponge in the detergent solution. Squeeze it over the furniture to cover the mossy areas.
Scrub the furniture with a scrub brush. Rinse the brush frequently with the hose to remove moss from the bristles.
Rinse the lawn furniture by spraying it with the garden hose. Inspect the furniture for remaining moss or stains from the moss--you may need to wash it a second time. Allow the furniture to air dry.