HELP, Sea Hare

[outacontrol]

I bought a Sea Hare from a fellow reefer today, brought it home and drip acclimated it for 2 hours and then put it in the tank. Soon after enter tank its "back" opened up and you could see a large white sack? about 20 to 30 minutes later the back closed up, its been a few hours now with lights off and the thing has not moved is this normal should I be concerned?
Help me, I am worried that in the morning I am going to have a dead Sea Hare and that it will poison my tank.

It is about the size of a pear. A good 4 inches long.