Can one's iMessage/FaceTime identities be hijacked at a public Wi-Fi?
Yes, this may be a security risk. But it's unlikely that someone hijacked your Mac using Wi-Fi alone.

In order for you to receive this message someone needs to login with your Apple ID credentials with a device that supports FaceTime (iPhone, iPad, iPod touch, any Mac).



Change your Apple ID password


Go to Manage Apple ID
Log in with your Apple ID
Go to Security > "Change Password..."
Change your password
Also turn on two-factor authentication if it isn't already




Check your Devices


Go to Manage Apple ID
Log in with your Apple ID
Scroll down to My Devices
Click any device that looks unfamiliar to you
In the new window click the blue circle with an x to remove it.




You may need to sign in with your new Apple ID password on your other devices in order to use all iCloud services again.