Its hard to tell precisely what the darker and whiter lumps are - to be honest, they look like cat or doggy doo as much as anything else. Even if it is a growing algae, moss, lichen or liverwort, they will not be dangerous to health, and a treatment which kills moss on hard surfaces should also take care of them, if they are living things.

I think the grey/white stuff is probably bird poo. certainly lots of cylinders of similar under the tree where the rooks perch.