escape the target mode
From your description, I believe you're looking at the firmware password:



This isn't specific for Target Disk Mode, and would have been set through the Firmware Password Utility.

This script may be able to decrypt your firmware password, however it doesn't work on all Macs.

python -c "print ''.join(chr(int(c, 16) ^ 170) for c in '`sudo nvram security-password`'.split('%')[1:])"


Source: CNET