WC473 which algae

I am changing the spin a little. We all have obscure plants and animals in our files that we haven't yet identified. So. . . what kind of seaweed is this?

I know the birders here can look at a single feather and tell you possibly age, gender, and species (post one and see!). I am not so sure that the insect and flora folks are, well let's say, so fanatical. This is an experiment, or test, if you will.