Bryozoans or Moss animals - read all about it:
http://www.liddiard.pwp.blueyonder.c...s/bryzoans.htm
www.st-andrews.ac.uk/~merg/bryozoans.htm
Makes great backgrounds for nudibranchs and slugs, which are often found "grazing" on the web-like structure of the Bryozoans. On your picture each little rectangle is actually a separate animal, which lives inside the calcium structure forming a "wall" around the animal itself. Some of them, like yours, form colonies and are the feeding grounds of many gastropods. The Bryozoans in turn often live on kelp, and feed on pelagic plancton with a small "arm" much similar to the little fan you see on barnacles.
In other words; if you find Bryozoans on the kelp, you are in a good place to spot nudibranchs. The Polycera quadrilineata for instance, feeds exclusively on the bryozoan Membranipora membranacea (common sea mat), which I think is the one on your picture.
That was todays lesson
