My shower is leaking, so I am trying to replace the washer, but I'm stuck!

I removed the outside shower knob, but cannot remove the one inside.

I loosened the screw using a long necked hex, but there looks like another
Screw inside. I tried different sized hexes, but they are either too big to fit
Inside the tube or too small for the screw. I also tried to pull hard in case
It is corroded, but the knob doesn't move!

Thank you in advance for your help!