You have a few options here...
1. cut the "trunk" near the base=using rocks to wedge the tree between so it grows elsewhere, or using a rubberband to adhere it to the rock till it attaches
2. use bones cutters to cut away the layer of rock right beneath the tree and then glue it anywhere you want.
If you go with option #1 you will have some chemical discharge from the trunk and base...but it shouldnt be an issue...however the small layer left will usually grow into another tree. If you go with option 2 there is no "damage" to the coral and very minimal discharge during the process as long as you handle the coral gently.
Hope this helps