Question
I have light green something (maybe lichen) growing on an orange tree.  I understand that it may be killing my tree.  What can I use to kill it or should I?

Answer
Lichens can be signs of a tree in stress, but that doesn't necessarily give any hint to what kind of stress.  Lichens themselves use the tree only for support. So, they are not causing any damage to the tree.
I would suggest that you fertilize the trees with a citrus fertilizer A slow release fertilizer is best. Check with your local nursery fro these fertilizers. A healthy tree will over come the lichens.