Moss and other organic matter on a wall is often the result water being driven on to the surface such as penetrating damp, however moss is not always a symptom it is also a cause.
Moss will grow pretty much anywhere that has enough water, so whether the moss occurs first or as a consequence of the damp from other reasons, it can easily exacerbate the damp issue. Moss can become dislodged with the flow of water, wildlife etc. and then builds up at points, for example blocking gutters which will either create these consequential issues such as penetrating damp or add to the problem increasing the amount of water striking the issue.
> Freezteq AM-10