Unlocking Droid XT912 with a Fully Broken Screen/Digitizer
In case any one had similar problem and unable to use the mouse, I saw an article while working to find a solution for my colleague's same type of phone. It indicated that the xt912 won't provide power to the mouse so you need to connect the mouse through a powered USB hub.  You can find it on Amazon easily but make sure it is powered with a power supply. Good Luck!