Brown algae, lower shore, cornwall. Small air filled blobs, attached at the base.
No interactions present.
I think this may well be Oyster Thief (Colpomenia peregrine).
Your snippet about dates is valid but March in Falmouth is early summer sometimes!
Lat/Lng: 50.14484, -5.0649
OS grid ref: SW811316
Gyllingvase Beach, Falmouth, Cornwall