They’re not freak-out-and-burn-your-tank-down bad. But they’re a nuisance and they’ll spread in numbers relatively quickly. They shoot out a mucus net-like thing to collect food and it bothers corals. Since you’ve got softies, they in turn may release chemicals in to the water to combat that and then you’ve got a mess on your hands. This is only hypothetical. You can super glue the tips and they’ll die. Some people crush their shells. I had them once and was able to irradiate them. I glued the tips and used tweezers to break the shells of the ones I couldn’t get glue to.
