Need to remove ball from shower head

I’ve been trying and failing for a long time to get this ball off the shower arm so I can install a new shower head.

I tried the following: 1) bought an adapter to just go over the ball, but it wasn’t big enough. I looked for another but even the guys at the hardware store known for good customer service didn’t know which product I needed

2) bought the the type of wrench that I thought would help me resolve this, but even when I place it one the flat side of the pipe part of the ball thing, it just spins around the pipe. It doesn’t grasp and turn.

3) removing the sticky filling between the arm and the ball piece.

Not sure if it’s corroded on. Should I give up and call a plumber? I hate to do so for such a dumb problem, but I feel that I’ve exhausted the limit of my skill set here. Any advice would be very much appreciated.