Question
We live in Northern Indiana. We have 8 larger trees on our farm, we have no other trees close to them. I do not know what kind of trees they are, but for the past 10 years since we bought the place we have noticed an odd green mold looking substance that grows on the trunks of the trees. The mold, as I refer to it is has raised higher on the trees over the many years, perhaps 20 feet or so.  It does not grow outward , it is a flat surfaced dark green that appears brighter dark green after a rain...its kinda pretty IF they were rain forrest trees...but they are front yard shade trees. I also wonder if it is something the kids should not be around. The trees are fairly healthy and leaf plentiful. They are seen occasionally in the area...and that mold can be seen elsewhere around here, just NOT this much. They are very tall trees that have been self maintained...the trunk base are about 3'-4' in diameter, and the leaves almost look like a maple, but not quite (not full enough). They have those small spinning droppings (us kids called helicopters) in the spring. I called the local nature resources to no avail...the once place I trusted most to call said others have called about that, but they have no idea what it is, but maybe to try some mild bleach/water mixture and see if that would take care of it...It would take alot of bleach to cover all the trees, plus I don't know if that would be safe. If any of this sounds familiar, or you have some ideas or leads I would like to hear from you. Thanks.

Answer
Sounds like a maple tree. The growths are lichens.

Lichens are an example of a symbiotic relationship between algae and certain fungi. They are capable of producing their own food. The algae associated with the fungus is a green or blue-green alga. There are three forms of lichens based on growth patterns. Crustose are species that are closely pressed against the surface of the limb or trunk of dead or live trees. Foliose forms are leaf like or prostrate but are also tightly attached to the tree. Fruticose forms are bush like, erect or hanging. Although lichens are found in most areas of Texas, they are most noticeable in areas that have extended periods of high humidity.

The effect of lichens on a tree are only slightly detrimental. The plants are epiphytes. That is they derive their nutrients from the air and not from the plant on which they are growing. Although they are not parasitized, literature reports suggest that lichens do have a slight negative effect. The main concern is that lichens give a tree an unkept appearance. Presence of lichens also is a good indicator of a thin tree canopy. This often leads homeowners to conclude that lichens are the cause and not the effect of thin foliage. The best control for lichens is maintain the tree in good condition. This will insure a dense canopy which will shade the limbs and reduce photosynthesis. Without photosynthesis, lichens are not able to manufacture food needed for growth and development.  

The baking soda recommendation I have herd of is 40 pounds of baking soda per 100 gallons which comes to 0.4 lb of baking soda to 1 gal of water. But I would not do anything about the lichens but I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. this will increase the overall health of the tree.