Corner bead cement covering
You just need to sand it down. Tile mortar is fairly soft and a common sanding block should do. Be careful not to scratch your tile (mask it if you like). Skim it with drywall joint compound if that doesn't yield satisfactory results, then sand it again. 

Next time, either be more careful troweling the mortar or apply masking tape to the wall first. You don't have to have mortar right to the very edge of the bullnose. In fact, you're better off leaving it back slightly so you can grout against the wall and have a clean look without mortar showing through.