A green mouldy substance on crabapple trees is a sign of illness in the tree. Applying pesticides or chemical treatments helps a crabapple fight off infections and disease that create the mould.
The most likely cause of green mould-like substances appearing on a crabapple tree is powdery mildew. Powdery mildew is green or white, depending on its developmental stage. It spreads by living on fallen leaves and spreading back to the tree in the spring.
A canker is the product of fungal diseases that enter the tree through a break in the bark or exposed wood, as from broken branches. One fungal disease will produce many cankers. Cankers are often oval-shaped spots that can swell or sink into the tree and take on a mouldy appearance.
Fungicides are essential for controlling both powdery mildew and cankers. Additionally, pruning severely affected branches, clearing away fallen leaves and irrigating the soil around the crabapple helps control the spread of fungal diseases.