Hi,
smichel0:
This morning nyx was still working but tor was down - now restarting.
The Tor daemon (instance) is unrelated to nyx, arm or other software that monitors your Tor relay. In order to verify if the Tor daemon is running on your system you should check the logs usually under '/var/log/syslog'.
Whow can I get rid of the arm-installation?
From your previous post it seems that you are running a system with the apt manager, the following command will remove arm from your system (given that you have installed arm via the package manager): apt-get remove tor-arm
~Vasilis