'MAC OS INSTALLER DONE' Pop Up, Can't Close

Try downloading this program which was written by Thomas R, a long time poster. The program will search for malware/adware The program will do the work for you which makes it easy. 

Malwarebytes Anti-Malware for Mac      10.10 and later

Malwarebytes uninstall

That's malware. Probably browser junk. Quit Safari or force-Quit Safari, and see if it goes away.

If it doesn't go away, maybe something previously installed.

An EtreCheck configuration report might point to any latent cleaners or antivirus or anti-malware that's been installed here, and MalwareBytes can usually remove that.

Download and invoke and copy the EtreCheck report to the clipboard, then open a reply here and then click in the (highlighted) button that looks like a printed page to get a text input box that's big enough to paste the EtreCheck configuration report here.

MacBook Pro 13" Mid 2012 can be upgraded to High Sierra, and to Mojave; to more current, and more secure software. Create a couple of external backups, check that your key apps work on the newer release, check that your printer has drivers available from the printer vendor, and upgrade.

Did you manage to fix it?? I have this issue now and Malwarebytes is not finding anything!! It opens Safari AND Safari’s preferences at the same time!

Thank you, I think I just fixed it using EtreCheck as well as resetting Safari.

I would not prefer to have CleanMyMac on the Macs around here. Have a look around the forums for some other opinions.