To answer the age old question, you need to understand what ball moss is and what it isn’t. It is not a parasitic plant that is pulling nutrients out of your tree. It is in the Bromeliad family (so are pineapples). Ball moss is actually an epiphyte, similar to many mosses, lichens, algae and even ferns. It can grow pretty much anywhere…trees, power lines, on a wall, gutter or even that old car that has been sitting in your driveway for a couple of years now. Epihpytes derive their nutrition from the air and only need physical support from their host. Ball moss spreads by tiny seeds, which are carried through the air by wind, rain, squirrels and birds, which makes it near impossible to completely stop the spread of it.
Ball moss has been know to cause some minor physical damage to its host (trees). If allowed to grow unchecked for a long period of time, ball moss can grow so thick that it inhibits leaf production. Basically ball moss blocks leaves from forming, so the tree then has a limited amount of foliage that it is relying on, which in turn means less food for the tree. Aside from that, ball moss causes no damage or harm to your tree, other than it being unsightly.
The best course of action with ball moss is to have it taken out of the tree when you have your regular maintenance pruning done. This should typically be done every couple of years, just depending on what species of tree you have. By simply removing dead limbs in the tree, you’ll get quite a bit of the ball moss out (usually 60-70%). If that isn’t enough for you, then talk to your arborist and let him/her know that you’d like as much of it taken out as possible. Just keep in mind that ball moss is not killing your tree and in 90% of cases, there is no action needed aside from regular maintenance.