apple alert scam for virus

Generally a restart will do it, but...

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

The free Malwarebytes may take care of it...

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

Read this

Effective defenses against malware and other threats

<https://discussions.apple.com/docs/DOC-8841>

Think about getting an ad blocker (I use uBlock Origin).

Think about changing your DNS to something like OpenDNS.com or CloudFlare's 1.1.1.2 which does some filtering.

Any luck yet?