Installer shows up at boot up disk

Hi

yesterday after I restarted my Macbook Pro, the computer wanted to install a new OS. I never asked to install it or installed any updates in the past. It was restarting just fine. So when the installer runs I get the message can not find resources for the installer. Running High Sierra 10.13.3.

so what I had to do is to restart the computer while holding the option key and select the MacHD drive to use as start up disk and the computer starts up ok. I tried to see if the Installer disk shows up in disk utility and under start up disk in system preferences , to see if i can remove it. but it does not show up. So how can i remove the installer as boot disk, as said never installed anything new in a long time. I do not want to run the update just in case the computer uses the installer again to run the update on.