I figured it out. I put a rubber pvc pipe collar on the pipe and used a clamp to hold it in place. The pipe no longer slips down plus you can’t see the collar.
On a side note, I found an easier way to prevent a “hot spot” on the pipe instead of trying to put a 1.5″ pipe inside a 2″ pipe and using Great Stuff to insulate What I did was purchase insulation tape and wrapped the pipe. Not only does it keep the pipe insulated, the aluminum backing is more appealing to the eye than a white PVC pipe.