Andy wrote:
We had our drive block paved last summer and are happy with the
result, no sagging or other problems as far as I can see. But
over the last few weeks of damp weather the drive has taken on a
distinctly green hue, less so where the cars are normally parked.
What is this? Is it moss or mildew or something similar growing
in the surface of the blocks due to being damp all the time? What
can I do to clean it off and stop it coming back?

Algae. Pressure washer or common chemicals will remove, but nothing will
stop it coming back.

Why not call it patina, and have a beer instead?