Usually these syllids have one very small tooth, sometimes more, but always so small that they can't penetrate your skin. Typical feeding behavior for syllids is to puncture the outer skin & suck out a bit of the coelomic fluid & then move on without doing any real damage. A few tend to stay longer in one place & fall into two feeding categories. The first one - which seems to hold most of these species - are grazers which feed on detritus, bacteria, dead cells, etc., living on their hosts which might be sponges or corals. The second group are those few species which actively eat their hosts by sucking in the soft flesh much like a person might eat a frozen yogurt cone. The trade-off for the host is that the worms will defend them against predators like nudibranchs and sea stars. In nature even the worms that directly eat their hosts probably don't do much damage but in the confines of an aquarium they can.
Boomer was right - I want your worm if you can catch it. There's very little real information on which worms eat which corals so I'd like to identify this one.
