Q: Can you please tell us what is going on with our tree? Most of that bump is hard as a rock but some of the edges are crumbly.
A: Your Japanese maple suffered an injury that caused internal trunk rot. Possibly from a broken limb that never healed properly and let water into the trunk.
Now the tree is trying to heal the wound by forming a callus layer over it. There is nothing you can do to accelerate the process except to keep the maple healthy.