I had often seen grey patches on fronds of laminaria washed up on the beach and had thought wrongly that they were probably fungal growth. However, I bought a new fold-out guide to the seashore and right in the front was a picture of a sea mat colony and realised that was what I had been seeing. Sea mats are small colonial creatures belonging to the phylum of animals Bryozoa. They can encrust seaweeds, stones and shells. They form a lacy pattern of cells and and in each cell is an individual animal or zooid which have tentacles that can be projected to filter food particles from the sea. The main predators of sea mats are some species of nudibranchs (sea slugs).
The sea mat Membranipora membranacea on a laminaria frond. Close up can see the lacy structure. The rectangular shape of the individual cells differentiate it from the other commonly found sea mat Electra pilosa which has oval shaped cells and forms an angular roughly star-shaped colony.