Although Spanish moss is most of the time not harmful to the tree, if it gets excessive it can become a problem as it can weigh down branches and potentially causes them to bend or break. If you feel it is a problem and want to get rid of it there are a few different spray solutions that should work well.
To make the spray:
Mix 6 tablespoons of copper sulfate with 1 gallon of water.
It is best not to spray during the growing season. Wait until the tree is in a dormant state so that there is less change of effecting the tree itself.
You can use a spray bottle for lower area or spray gun to reach higher or more difficult to reach areas. Apply the mixture to the moss until it is quite wet.
The process should be repeated several times over a few weeks.