Can my Find My iPhone password be different than my Apple ID password? I would like it to be different so my girlfriend can see where I am but not be able to access my entire phone.
In short, no.
Your alternative would be to use the Find My Friends app, which should be already installed in anything since iOS 9.
If you’ve removed Find My Friends or would rather not use it, an alternative is to share your location via the Messages app. Since you and your girlfriend probably text each other, this is a very convenient option, especially if you’re not going to share your location with anyone else.
Open her conversation thread, then tap the ⓘ in the upper-right corner.
On the next screen, tap Share My Location. You can choose from several options, including Indefinitely.
Now if she opens her conversation thread with you, she will see your location by tapping that same ⓘ
Note that Messages and Find My Friends share this information, so she would also be able to see your location in Find My Friends (including in the Today View widget).