Blatently stolen from another site - Rob Toonen is the author of this particular article.
"Because most of the members of this family (and the serpulids described below) reproduce sexually and produce planktonic larvae, there is little risk of them reaching high densities in a reef tank, but there are a few species (such as the "duster cluster worms," Sabella melanostigma, pictured here) that are capable of asexual reproduction, or produce such a short-lived, nonfeeding planktonic larva that they are capable of reproducing within a reef tank. Such species can become so abundant that they literally overrun other animals in the tank, but if they do so, it is likely a symptom of a nutrient problem in the tank. "
From what I have read the big ones you are talking about are the serpulid type, the small ones that we get all over our rock is the sabella melanostigma.
HTH