Unable to install an ipa App with a strange gray icon

Hello,

I am unable to install an App provided by a developper which is not downloaded from Apple Store. The App is called TIXCHANGE. I use iTunes, menu "Add File to Library" and I click on Sync. Once the App is uploaded on my phone, the TIXCHANGE App has a strange gray icon with circle. The App does not work and there is an endless "Installation" message. Why? What is the meaning of this gray icon?

My configuration is iOS 8.3 & iPhone 5S, iTunes : version 12.1.12 x64 on Windows 7

Thank you

Jerome