This is a famous indicator I have seen used by many Forex traders in MT4 platform. This indicator uses the difference between EMA of "close" and EMA of "open" to form the oscillator lines (not just a convergence/divergence calculation between EMA of "close")
Current market state is determined by which histogram is above zero line. For more confirmation, wait for it to cross above Signal line too. Crossing down the signal or zeroline should be the exit condition. Also, look for divergences.
Use plot labels BullHisto/BearHisto/BearLine/BullLine/ZeroLine/Signal for setting up alerts.