Hi. Noticed something weird in my tank 2 days ago. While cleaning the glass I noticed some slug like thing sticking its head out from one of the live rocks. At the same time this thing was "blowing smoke rings out" at every two to three seconds. Not to sure if this would be toxic for the fish, I grabbed the rock out of the water and tried pinching it out. But it just went deeper in. Still determined, I managed to get the head out, which had a hard shell like covering and subsequently the body, which basically felt and looked like a pickled gerkin!! It may have been about 4 to 5 cm long and about as thick as an average pinkie. Sorry was not able to photograph. Any ideas? Should I be concerned that there are more of them?