Is there a special tool to remove this shower cartridge cover?
Does not appear to be any &quot;special&quot; tool requirement per these pictures.
Put a strap wrench on the outermost silver bit and unscrew, or (judging the beat-up look it already has) just scar it up some more with slip joint pliers like the last guy did, at a guess. If the central part turns when you do that, put a wrench on the flats to prevent it from turning as you unscrew the silver part. If the silver bits are more conjoined than it appears, you might need to take the screw(s) in the wall out too.