My first assumption is that this port is not opened by Cloud9.
Reading their documentation kind of confirms that: https://docs.c9.io/docs/run-an-application
Available ports on a hosted Cloud9 workspace If you're developing a server application, please note that you need to listen to 0.0.0.0 ($IP) and 8080 ($PORT). Listening to this port will enable your app to be viewable at http://-.c9users.io
You can also bind to ports 8081, and 8082, which can be accessed by http://-.c9users.io:8081 and http://-.c9users.io:8082 respectively.
Please note that 8080, 8081, and 8082 are the only available ports on a hosted Cloud9 workspace.