it's about the size of a quarter with all the outstretched tentacles. the actual body is probably about a 1/4" long, though most of that may actually just be a poop trail.
i have some in my tank as well.. they don't seem to bug anything in my tank they are always in the sand bed.. heard that they are not bad. and are good for the tank! but I could be wrong. hopefully someone can answer ur question. because if they are bad ill start picking them out.. but ill wait to see what other reefers have to say!..
Generally speaking they're a sign of excess nutrients. A normal, low nutrient tank shouldn't have them or at least not visibly. Like most worms their tentacles can sting some coral.
I be got spaghetti worms in one of my tanks. They don't bother anything, but they're red. They do go on the glass sometimes. Like a lot of things, they do multiply when nutrients are high and there's excess food. I find them annoying, but harmless.
I had a spawning of the shell-less snail crap, and now there's like a zillion of them on the glass at night. Maybe they'll Duke it out with the face huggers.
it's about the size of a quarter with all the outstretched tentacles. the actual body is probably about a 1/4" long, though most of that may actually just be a poop trail.