Filling joint between backer board and wall in shower
You don't want to use joint compound that close to the shower. That gap at the top probably goes all the way to the bottom behind the door track and should be fixed too. Remove the door track and scrap all the caulk off everything: walls, tile, door track. Get some epoxy grout from your home store and spread it from the bottom of the shower all the way to the top, filling in that space between the wall and the tile. Follow directions for the grout. Once the grout hardens reinstall the shower door track and get a good paintable caulk and run beads on both sides of the door track and at the bottom. good luck