Remove "steamclean" application
Open the Terminal application and type (or copy and paste) the following command and hit ENTER. This will remove the steamclean job and should stop those popup messages.


  launchctl remove com.valvesoftware.steamclean


You may also want to run the following command to check for other Steam related launchd jobs.


  launchctl list | grep steam


On my own Mac there is another called "com.valvesoftware.steam.ipctool" that you may also want to remove.