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
"Prolonged erection greater than 4 hours and priapism (painful erections greater than 6 hours in duration) have been reported infrequently since market approval of VIAGRA. In the event of an erection that persists longer than 4 hours, the patient should seek immediate medical assistance. If priapism is not treated immediately, penile tissue damage and permanent loss of potency could result."