Unscrew the grub screw completely, it might still be holding the bracket even though you have unscrewed it a bit. There's not much to it, if that grub screw comes out then you should be able to remove the accessory before removing the plate behind.
Have you tried getting something to lever it off? You could even try a pry bar or similar as long as you have a piece of card or thin ply between it and the tiles.
Unscrew the grub screw completely, it might still be holding the bracket even though you have unscrewed it a bit. There's not much to it, if that grub screw comes out then you should be able to remove the accessory before removing the plate behind.