What is this icky huge slug?? **PICS**

[BCShelly]

Last week I lost 2 fish and found no remnants of them in my tank. I set a trap and caught nothing. Then today I come home and found this slug peeking out from under one of my rocks. I quickly got him out and he's currently floating in a plastic bag. He has this really weird grabby hand like thing on his butt, you can see it in one of the pic's. He is black with a purple outline. Any idea what this is?? Should I get rid of him? I wonder if he's the midnight fish killer. Thanks! All the help you guys give is great for us new salt water addicts!