While exact identification is difficult from the picture, it is either a whelk or some form of predatory, shell drilling mollusk. First it's clams, then its LPS's and so on, and so on ....... Find them and euthanize them by putting them in a paper towel inside a zip lock bag and freeze them. Do not flush them down the toilet. Who knows what these things could do if they survived and made it into the salt line of any major river.