This is a spongy moth catepillar. You can spray  with BTK an organic insecticide. You can wrap the base of the tree with duct tape with the sticky side exposed to prevent them from moving up the tree. However, there are so many even if you kill some they can come over from your neighbors. They will mature and eat the leaves of some trees causing much damage but healthy tree seem to recover after they have gone through their cycle. 

We've attached a fact sheet with additional information about the spongy moths.

Best

Your Friends at CCE