My MacBook Air is automatically connecting to a password protected network that's not mine! I'm starting to think I've been hacked because my Macbook Air is frequently jumping to a neighbors password protected network on its own!

I see the network in the "preferred network" list and I don't know how it got there!  I also don't know what that network's password is but my macbook does!  To make matters worse I've also seen my iPhone on the same network!  Anyone have any ideas as to what's going on?
Wi-Fi passwords are synchronized via iCloud keychain. That's why your iPhone also knows the password. Try deleting this network from Preferred Networks.

Also delete item from your keychain.
In Keychain Access.app select iCloud keychain, find the item with the SSID (network name) of your neighbor's network and remove it (Right click -> Remove).