We have detected a trojan virus (e.tre456_worm_osx) on your Mac.

Any such popup message will be a scam.

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Stedman1 has this covered if it is occurring in your browser. If you are receiving this message because you are running any anti-virus application on your Mac, then follow the product vendor's removal instructions. Anti-virus products adversely impact the normal operation of the Mac operating system, are worthless, and give false positives that can raise your blood pressure.

Instead, you might consider the free Malwarebytes to interactively, and occasionally scan your Mac for anything (non-virus) that has infiltrated it from wrong buttons clicked on a web site, or aggregator download site.

These yahoos will do everything they can to get you to click ANYWHERE on the popup .. don't do it. But don't just ignore it either .. eventually, you'll likely accidentally click it. When you get an unexpected popup, the best thing to do is NOT TOUCH IT, open Force Quit and force quit the application. Then when you reopen the app, if the popup returns or you can't force quit the application, shut down the computer and then restart.