Hi,
What does the console log say when you launch the app from FLP? Perhaps there's an unhandled error coming from the app itself.
Regarding executing the app directly from HCP, make sure you have an HTML file and that the path to it is configured properly in the neo-app.json's welcomeFile property (see more info: SAP HANA Cloud Platform).
There's no option to 'unregister' from Web IDE, you can however delete it from your FLP directly.
Regards,
Michal.