If you did not install it, you are OK. For you to get this thing you have to give it an administrator’s name and password. You are running as a standard account, aren’t you? If not, open system preferences, create a new account, make it an Administrator account with a strong password you won’t forget. Then log off your every day user account and log into the NEW administrator account, change your every day account to a standard account, turn on “Fast User switching” at the bottom of the user list window, lock the preferences, then LOG OFF the new administrator account, and then log back into your usual account to continue surfing as normal. You can still install stuff but it will require both the admin name and password be typed in... It’s an extra layer of protection. Oh, don’t use “Admin” as the name of the Administrator account.
Thank you! It was a scary moment, because it was exactly the sort of thing that happens on PCs and you can’t click anything without installing whatever the bug is.