is this a scam ?

Yes it is.

1. Force Quit Safari .

Press command + option + esc keys together at the same time.

When Force Quit window appears, select Safari .
Press Force Quit button at the bottom of the window.

2. Relaunch Safari holding down the shift key.

3. Go to Safari / Preferences / Security /Privacy

Cookies and website data:

Click “Details” button.

Remove the cookie related to this, if there is one.

4. Download and run mac antimalware tool : https://www.malwarebytes.org/antimalware/mac/?utm_source=blog&utm_medium=social

You can also find some info here:

http://www.adwaremedic.com/kb/scampopups.php

Yes that is a scam.

Never use any kind of "anti-virus" or "anti-malware" software on a Mac. That's how you create problems, not how you solve them, and this problem has nothing to do with malware.