Spyware

Try double clicking the Home button or swipe up from the bottom of the screen and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.

Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch

Safari website data on your iPhone or iPad - How to clear

No, it is a scam. Close the offending page, and avoid sites that are involved in such bogus advertising tactics.

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