I think it is Either Enteromorpha intestinalis or linza.
No interactions present.
I have added agreement to 'Enteromorpha intestinalis', but Enteromorpha is combined with Ulva these days.
So correctly, it is Ulva intestinalis.
Lat/Lng: 50.6115, -2.1318
OS grid ref: SY907790
On a rocky beach in Dorset