There are multiple parts to this answer. First, everything stored on the file system of an iOS device is encrypted, provided the device uses a passcode. Hence with respect to the data actually on the iOS device, provided you're not trying to hide the data from the user who knows the passcode, you can have some confidence that core data is secure.
However just to be clear, Core Data is not actually a way of storing data. It is a common interface to an object store, which is usually backed by sqlite on the device's file system.
As pointed out by André, this sqlite file may be backed up, leaving it exposed to prying eyes.
Apple's recommendation (that I agree with) is that passwords should only ever be stored in the keychain, which is easy to do, and allows you to control when the entry is accessible (ie. while the device is locked, after first unlock, etc.).
Other apps can never access your app's data in iOS, irrespective of where you write it (unless jailbroken).
Here's a Ray Wenderlich tutorial that walks you through doing this with swift: https://www.raywenderlich.com/92667/securing-ios-data-keychain-touch-id-1password