Red Algae can be a menace. On houses it can cover the walls in very little time, and isn't easy to treat. I think it arrived in this region about 30 years ago, and has since become quite a problem.
The Algae is a microalgae (Trentepohlia) which usually starts life as green, then turns red with age. I have seen new houses around here that are covered. The Algae seems to like cement rendering more than actual stone.
Usually it's an unsightly mess, but on my poolside urn I rather like it.