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

[Black Phantom]

Looks very nice from the pics. Maybe a Rhinophore but hard to tell. Don't let it die as I'm sure they just eat sponges and such. Thats why they are so hard to keep in captivity, not enough food source.
As far as losing fish and not finding them - the rest of your fish probably consumed their tankmates if they died. Anything new added to your tank lately to cause a couple of fish deaths?