While installing the latest version of our app (outside of Google Play), a pop-up message shows up:
Installation Blocked This app can collect data that may be used to track you. Even if you have heard of this app or the app developer, it's still dangerous to install an app from an untrusted source.
so
What can make Google Play to detect an app harmfully?
How do we understand what's Google Play problem with our app?
after asking form google support about my problem they said to me that because I using sensitive user data I should use SSL in my communication with my server and after update my app that message disappeared.
also, read this link it has great info about this kind of problems by google protect.
and if you think that google protect unreasonably saying your app is harmful you can use this link to inform them