The Hue Motion Sensor cannot reside on your Hue Bridge and SmartThings hub at the same time. It’s one or the other.
In order for you to have it discovered and added to the SmartThings hub, you need to reset the Motion Sensor so that it goes into pairing mode. Then you can Add Thing in the ST Mobile app.
Once it has been added, you will need to follow the instructions on creating the custom Device Handler in your IDE and then once you have created / saved / published the Device Handler, you will edit the Hue Motion Sensor in IDE and assign/modify the Type to “Hue Motion Sensor” and Update it.
Your Motion Sensor should now work in SmartThings.
EDIT: Not knowing your knowledge of SmartThings, if you need assistance to get started in the right direction. You can read this thread to give you some help in creating / publishing a DH from existing code.
FYI: When logging into IDE for the first time, always use https://account.smartthings.com (do not use graph.api… or any other url published in anyone’s docs as things in the ST world have changed).