That worm may seem to be smooth, but it's definitely a Polychaete of some variety (you can see the parapodia - the places where the bristles leave the "bristleworm" - in the pics). That bottom pic shows some cephalic tentacles (tentacles on the worm's head), but I can't see exactly how many.
As a general rule of thumb - 4 tentacles = Dorvilleid worm (harmless/beneficial); 5 tentacles = Eunicid (may be harmless/beneficial, or may be predatory).
In this case, I would guess your worm there is harmless.