If you’re using an Android device, the beam brush can only be recognized by one connection at a time, so right now your phone itself is taking up that connection.
To fix this, go into your Bluetooth settings and forget the brush. Then turn Bluetooth off.
Next you'll need to reset the Bluetooth cache. To do this, drag the app icon from the app drawer up to where it says "App Info" and choose to force stop the app. From there you can turn Bluetooth back on and start the app again.