Identifying Pets or Pests

One huge difference between marine and fresh is the abundance of "critters" that seem to suddenly appear in an marine tank with live rock. One piece of rock seems to be growing some beautiful red seaweed but more surprising was this little guy...

It appears to be an incredibly small but exceptionally long tube worm of some kind. It definitely has a mouth part made of four opposing white (hairy) extensions and continuously crawls over rocks and sand in search of food. It also has the ability to coil itself up like a spring as shown above.
Once I know what this is, I'll post a reply, hopefully it wont be an issue for the seahorses - nor end up fodder for the Camel Shrimp.