Sometimes if an application isn't in the exact same spot or has the exact same name, MIDI Mixer will see it as a different program. And to my knowledge, for MIDI Mixer to see the program, it needs to latch into Windows audio and register there (AKA be seen in Windows Volume Mixer) and sometimes MM needs to restart to refresh its list.
So the application is probably greyed out because MM has a different path of the program (is it installed in the same location?) or the program has a different name (my Spotify had this issue when I had it installed it via the Microsoft store. The .exe would be Spotify[version number].exe so MM would not recognize it as Spotify)