Last week Codium sub sp atlanticum (green sponge fingers) was found at Dale Fort Jetty. It gave itself away with a display of blunt utricles (not a euphemism but microscopic structures at the tips of the fronds).
The other species was not uncommon but found in a most unusual place. Himanthalia elongata (or thong weed, again, not a euphemism) is usually found low down on quite exposed shores. Elsa, one of my students from City of London Girls school found it up on the middle shore at Frenchman's Steps Bay, which is very sheltered. People who've been to Skomer will have seen Himanthalia if they got on or off the boat anywhere near low tide in the summer. It has long dangly structures (receptacles; its reproductive equipment) that only grow in summer. The weed casts these off in autumn and overwinters as a small mushroom shaped structure. Some of the Frenchman's Steps specimens had some bedraggled receptacles attached, probably survivors from last year.
I think it persists here because the shore faces north and it has overhanging trees making it cooler, dimmer and damper than this height of shore might be expected to be.