oh man, there's a huge amount of stuff out there about these guys. if you pull em off theyll reproduce more where they are. Theres a couple different things thatll get em. Peppermint shrimp eat them, copper banded butterflys; raccoons i think? Or you can squirt kalk paste in their mouths. myteemouse, just covers em in kalk paste, or squirts em with boiling water- thats worked for him. or lemon juice.
But yes you definetly want to get rid of them in there, because they will spread like crazy! we had 1 tiny one, now three months later we have around 13. our fish attacked the peppermint we added. we're thinking we'll try the kalk next!