Not Coral, But...?

Out to blow away the cobwebs in Tyninghame, with a walk along the spit of Belhaven Bay. We saw a few pieces of this but I'm not sure what it is. It may be dead calcareous seaweed but, whatever its name it is rather pretty among the innumerable shells and the colourful stones of the beach. 

Edit: And guess who forgot it was a milestone???