When I think of strings, I think of the spaghetti worm. This worm lives in mucus tubes surrounded by rocks or sand generally in the intertidal areas and uses its thin white tentacles to gather food. The tentacles for a worm this small would probably be a little under a foot in length, while a worm with a body length of a foot will have tentacles about 3 feet in length. The tentacles themselves have a little groove filled with cilia (hair-like structures), that are used to move food particle from the sea floor to the mouth.
As an added bonus, you can also see a string of nudibranch eggs (the thick white wavy line to the right of the worm).