That icon is the placeholder icon for apps without an icon. Since you deleted your phone there's no way to find out what it was; might just be the result of a

That icon is the placeholder icon for apps without an icon. Since you deleted your phone there's no way to find out what it was; might just be the result of a bug in iOS, or some test app you might have installed at some point. Or it could be a spy app -- but to install it, your phone would need to be at least unlocked (I don't think iCloud password is necessary)