That is correct; the HTTP response is 200 telling you that the server responded with some data. The response data is a JSON reply which contains information about the JSON request. You need to contact the owners of the website, or the documentation, to find out why it returned the "Invalid access" status.
The best way to achieve this will be to install a server application on your own device, and have a client-based application on other devices. Then upon connection, detect the devices and connect to them using server-client channel that you will hold in the network.
This is the safest way, and the simplest way, otherwise USB won't allow much of permissions, such as executing any command (write commands are way too strict).
By inspecting the Android sources to find which module is generating the message (if the message is not prefixed with the module name). Once the module is known check if it provides some kind of notification that can be used.