Possible infection on a Mac, specifically Safari

When I open Safari I get a screen  headed http://mac-system-issues.info which says that the mac is infected.  It gives a number to call - 844-806-6841 - and a long alphanumeric string to give when calling.  Various phrases in the text would indicate that it is bogus.  Safari is frozen and can only be closed by Quit Safari.

The website that was visited just before this happened is legit - it is the Episcopal Diocese of Maryland.  I have successfully visited it with Chrome since this happend.

Nothing else on the Mac seems affected.  Quitting Safari and Restarting the Mac did nothing.

Googling http://mac-system-issues.info brings up a screen that says it is part of a verification that Apache http server is working OK.

I called the phone number.  It was answered by voice mail, which after a while said that my call could not be taken at this time and to try again.

Two questions:

Does anyone know what this is specifically?

More important, what is a good AV program to run on a Mac that might get at this?

(In many tens of years working on PCs, this is my first encounter of something like this on a Mac.  I have quite an arsenal of Windows tools, but no Mac ones.)

Ron Hicks