With this video shows to move a part of a tube in a part. What I did find out in in help was to check the detached plane to move. Also, I guess with a construction part nothing can be done. True? What is the large circle from?
I have no idea what the large circle is from, but you are not performing a move. You select the mirror command, select one set of faces, and proceed to attempt to mirror it about a face you seleted. I do not believe is your desired outcome, but from your description, I'm unable to figure out what you are attempting to do. If you want to post the model and provide very explicit instruction of what you are attempting to do, someone might be able to show you how to do it.
Production: ST10 MP7 Testing: SE 2019 http://Grundey.blogspot.com