Is it possible that you have LastPass installed? It started happening a day or two ago, which is LastPass was last updated, and it makes sense because they added support for Samsung GS5's fingerprint scanner: LastPass update adds support for Galaxy S5's fingerprint scanner | Android Central
What I don't understand is why it happens on a Moto G and only when I rotate the screen back to portrait mode..
I can confirm it was that, they released an update with the changelog "fix fingerprint overlay appearing when it shouldn't". I updated and it doesn't happen again