iPhone compromised notification on Safari

It is a popup from the website that was visited on the bottom of the page. You are still able to navigate away from the page by tapping in the search bar and entering another web address. That website got paid to pop up the message to scare you. The website is not a legitimate website, I went there to confirm and another website probably redirected you there.

Swipe upwards about half way from the bottom of the screen and pause at the middle. That should bring you to the screen with all the apps showing. Choose the app you wish to force closed and swipe it all the way up. Here is a set of instructions from Apple. Close an app on your iPhone or iPod touch - Apple Support

Force closing is done by swiping halfway up from the bottom of the screen and navigating to Safari and swipe up on it to Force Close it, but when you launch Safari again it will open up to that page as that was the last one viewed so that is not really an answer for you.

You have several options and any one of them will work:

• Tap in the Address field on the bottom of the screen and type something like "apple.com" to take you to a different page.
• Tap the tab icon on the bottom right of the screen that looks like 2 squares and then tap the "x" on the page that looks like that one and any other pages you don't want as a tab. The page that directed you to that one will need removed as well. You can "x" out all of them and it will not be a problem.
• Reset Website data at Settings > Safari > Clear History & Website Data, a prompt will also come up to close all tabs and you should choose Yes.

The phone is not hacked. It is a spam message. Force Close Safari, it should go away.