Photo: Dr. Wayne Nishijima
Lichen is very common on the bark of citrus trees. It does not harm the tree in any way, but if it bothers you, you can either peel it off or spray it off with water. If you spray lichen off with water, do this early enough in the day that the trunk of the tree will have time to dry before nightfall. This is to avoid spread of fungal infection.
Kendal Lyon, Hawaii Island Master Gardeners