When nerites breed, the male gets on the female and they fit together, he then extends his reproductive organ inside her shell and wraps it around. So I think that is his organ, stuck outside because that is what it looks like when it's extended out, which normally you wouldn't have been able to see. He had a malfunction :P
Cool snail expert. So you're saying my Nerite trying to mate with Spixi? I wonder what's the end result going to be.
Also I try pulling its "thingy" with a tweezers hopefully that's not the cause of losing his manhood.