How to know if my phone was hacked
How to know if my phone was hacked and what to do about it, such as apps to alert me to hack attempts
iPhone 6s, iOS 12.0.1
It could be a false alert, 'cause your iPhone can't be hacked unless it's been jailbroken.
In any case, the best thing to to do is to change your Apple ID password. See this support guide:
If you think your Apple ID has been compromised - Apple Support
Regards
Giulio