Apps to lock screen asks for password after locking I have a Moto G4 Plus, with fingerprint scanner fingerprint reader that is able to unlock(physical button) the phone by just giving the fingerprint, no password is required.
I'm trying to implement a "double tap homescreen" on my phone, so I used Nova Launcher to enable that feature.
It locks the screen, but the problem is that when I try to unlock the phone using fingerprint after locking by this way, it asks for my numerical password, instead of just unlocking directly. 
I tried other apps to lock the phone with double tap, but it does the same thing.
How to lock screen with these app and no being asked for a password after that.
This is a thing to do with the Secure/Smart Lock setup.  When using third party apps the system will default to the "primary" method (whatever you set as backup to finger print whether it is PIN, Password, or the draw on the screen gesture lock).  The only way I have seen to get around this is to grant root access to your Home Launcher (Nova does allow this in advanced settings), but it is not a guarantee with every device.  Here is what I found doing a quick Google search per your question: 
http://feedback.novalauncher.com/forums/145180-nova-feature-requests/suggestions/12241728-allow-lock-screen-function-to-be-unlocked-by-finge

Which does seem to indicate it is definitely more of an issue in Marshmallow (and probably future releases).  Hopefully enabling root MAY get you around this problem.