If you saw that fake message while browsing the web with Safari, the website that is promoting that Ad intentionally did not provide any close button, but you can still navigate to a different page in Safari using the Address Bar. If you close Safari when that page is on the screen, then it will still be there when you open Safari again until you go to a different page. That is default behavior for Safari to return to the last visited page. There is no need to shut your phone down in that case.
Also be aware that some third party apps/games you play on your phone may also include Ads if the developer chooses to monetize their development in that way, often times when they are not charging the user at all. You are still able to close the app in this case though without requiring a restart.
Please remove CleanMyMac from your computer using their uninstaller before you run into any of the problems reported by others here. You just need to use the Search option on the top of the page and enter "CleanMyMac" to understand why it is called "BrickMyMac" around here. Some have had to completely wipe their computer and start fresh again due to the damage it had caused.