In short, the answer is no to the question – “Is it killing my tree”? Spanish moss and ball moss are two of Florida's 16 native bromeliad species. Both are epiphytes, or air plants. Epiphytes can attach root structures to their host plant, but, they do not parasitize this plant; they simply use it for support. If air plants become so thick that they shade the leaves of the tree, growth could be slowed down. You usually see more air plants on weakened or damaged plants because they may also have thinner foliage. This allows more light into the branches, thus stimulating the growth of the air plants. So, air plants grow faster on stressed trees because the trees are weakened, but do not cause poor tree growth.
Spanish Moss (Tillandsia usneoides) is found hanging from tree limbs, especially live oak and cypress. It is gray when dry and light green when wet. It can hang down from tree branches in streamers up to 20 feet long. The small flowers are pale green or blue, and fragrant at night. Stems and leaves are slender and curly. Spanish moss has no roots; the leaves catch water and nutrients from moisture and dust in the air. 
Ball Moss (Tillandsia recurvata) is gray-green and found on tree branches or telephone wires. It is often mistaken for a small clump of Spanish moss. It grows in clumps 6-10" in diameter on most kinds of trees. Tiny seeds are blown by the wind until they land on a tree branch. They stick fast and develop root-like attachments to the outside of the bark.
Ball moss is able to convert nitrogen in air into a form that plants can use like fertilizer. Except for beans and peas, most plants cannot do this. So, when ball moss falls to the ground and decomposes, it provides a little more fertilizer for other plants.
For more information about Spanish and Ball mosses, please access the UF/IFAS Extension publication Florida’s Native Bromeliads at: http://edis.ifas.ufl.edu/uw205.
Both photos are courtesy of Ed Gilman, Professor at University of Florida/IFAS
