Hello Candace, the green moldy spots you describe are likely a combination of Green Algae and Lichen Moss growing naturally on the trunk of the tree. Both like moisture and cooler temperatures and most often found on the north side of the trunk, but can be all around the trunk at certain times of the year. The other challenge you have is Apple Scab, a foliar disease that causes the leaves to drop prematurely each year. A multi-purpose fruit tree spray will contain the right fungicide (s) to control Apple Scab. Treatments start just as the buds begin to swell in early spring and need to continue untill the leaves are nearly full grown on a regular schedule per the label instructions. This should greatly reduce the leaf loss on your flowering crabapple
Sincerely, Richard
