Whenever the MACD indicates a trend reversal (from bullish to bearish or vice versa), this indicator displays a gray horizontal line from this moment until the moment the trend is reversed, and then paints the space green/red between the gray line and the price according to the indicated trend
It is important to note that the grey line acts like a support/resistence. And therefore:
If the indicator shows an upward trend, but the price breaks the gray line drawn from top to bottom, this may indicate a reverse breakout. A similar case applies to downtrends also.