Development of HallMonitor seems to have stopped since version 0.4.2 (latest commit of the author "durka" is from 22 Jul 2014). This version doesn't support CM12(.1) (see issue 52 on durka's GitHub page).
Installation of the "MagnetOff" Xposed module is not a very useful workaround since you can't use the hall sensor (magnet sensor) anymore and would have to fall back on the proximity sensor (which is a pain in the a**).
The GitHub user "manusfreedom" has added several features and took development up to his version 2.0.2 beta (but his last commit is quite old too, from 21 Sep 2014). Has anyone tested manusfreedoms fork on CM12(.1)? Or does anyone know about a revival of HallMonitor's development? Would be so awesome to have this for CM12.1