Certain Android apps are designed with phones in mind and didn't expect themselves to be used on an Android TV box such as NextD. As a result, the apps developers may limit the target devices and display the "incompatible" message in Google Play store.
However, since NextD emulates the touch screen, motion sensors, and the users can plug in a webcam with microphone, most of the aps displaying the "incompatible" status are actually usable on NextD.
Although these apps are not accessible from the Google Play store, the user can easily find and install them from the pre-loaded 1 Mobile Market app store on NextD. The Amazon App Store can serve as an alternative source as well.
If you cannot find the Android app you are looking for from the alternative Amazon App Store and 1 Mobile Market, you can also consider looking for it on the Internet and download the APK installation files through a browser.