Malware Pop-up message

- Quit Safari and reopen with Shift key depressed.

-The popup could be caused javascript on the website. If that is true you can disable javascript but many site need javascript to work

- You can check to see if adware is on your computere:

First try

How to install (Really remove) adware

https://discussions.apple.com/docs/DOC-7471

Stop pop-up ads and adware in Safari

https://support.apple.com/en-us/HT203987

Adware Removal Guide : Identification

http://www.thesafemac.com/arg-identification/

Next if necessary:

Malwarebytes Anti-Malware for Mac

https://www.malwarebytes.com/antimalware/mac/

https://www.malwarebytes.com/mac-download/

You have a junk app called Mac Adware Cleaner, made by a company known to produce malware, on your computer. Malwarebytes Anti-Malware for Mac will remove that, as well as any other junk or malware that may have been installed along with it.