It means there are still some files from the old Facebook in your phone or you just already have the same app installed.
1) Reinstall the orignal Facebook/Messenger
2) Go in settings, then Applications manager
3) Go until your original Facebook/Messenger (pict.2)
4) Delete cache and Data (pict.4)
5) Uninstall your original apps
6) Restart your phone
7) Install The Hello Kitty Facebook/Messenger
If you have your facebook or messenger built-in, please check my post how to install a modded application.