I have a bunch of these in my tank, and I just happened to see one releasing its stuff tonight. I was watching the tank, and I noticed what looked like smoke inside of it. I then saw one of the slugs releasing it into the water. I looked to be coming out of a tube next to his feelers. Strange.
I have noticed these in my tank for about 3 months, at first I saw just one, now I bet I have close to 20 small ones in there.