Cannot undo tap/Faucet to replace washer
Clean off mineral deposits with a brass or steel brush and a lime dissolver such as CLR or Limewaway. Towel dry as much as possible.  Dry further with hair dryer in an attempt to evaporate as much moisture as possible; this, to leave room for penetrating oil to penetrate.

Aply penetrating oil such as PB Blaster or Liquid Wrench.  Tap, tap, tap the the valve to set up vibrations to encourage the penetrant. A vibrator can be useful to assist, such as an electric tooth brush or electric engraving tool. Allow penetrant to sit for hours and tap some more after that.

Use a tight fitting wrench or possibly a pipe wrench to turn it anti-clockwise.