If it's segmented, it's an Annelid, and likely a polychaete (bristleworm of some sort). I've never heard though, of one feasting on algae.
Actually, I don't know of any worms (except some nudibranchs) which have a diet of algae.
Well, hate to do this to you, but here are a few links to marine polychaete ID. Hopefully you can pull something from on of these!
>The Large Worm Turns…< >The Worms Crawl In…< (look closely at the syllids near the end of this article.
>Polychaete Annelid Identification, or “You Can Always Tell A Bristle Worm…< >Bristle/Fireworms Identification FAQs < >Bristles, Feather Dusters Fire and More: Polychaete Worm Diversity< >Phylum Annelida -- Polychaetes, "the bristleworms"< >Marine Worms< <---- This is in
.pdf format (you need Adobe Acrobat), but it's a good document.