unwanted pop up after login
recently, upon logging onto my MacBook Pro (using Safari) and before I even go to any of my favourites, I get this annoying pop up which slides quickly from the top right corner of my screen and then disappears immediately. The first time, I erroneously clicked on it and it was a sex invite with lewd photos! I am getting these every day now and of course I don't click on them but they do show different names of women inviting sex! Has anyone had these and could help me to remove these unwanted pop ups? Perhaps I have a malware or virus that should be removed but not sure the best way to do that. I'm using Mac OS HighSierra Vers 10.13.6 and Safari Vers 13.1.2
Many thanks
MacBook Pro 15″, 10.13