Robart's Library
Background
This is a cast-in-place, hammer textured concrete wall which is experiencing uneven rain washing. The light areas receive sufficient driving rain to wash off the grime. The dark line separating the washed and unwashed portions represents dirty wash water accumulating and drying in-situ, leaving additional grime behind it.

In this case, the only concern is an aesthetic one - the grime will wash off if a gentle, misting spray is applied for a sufficient length of time to break loose the dirt: the ivy can then be saved. Or, why not accept graceful weathering of buildings?