what is this?

[uring]

I found this in my tank a few weeks ago, but he's only out in the dark...so I only saw him when the tank was turned on, or if I went poking around with the flashlight...I had a really hard time getting pics.
Last night I was poking with the flashlight and saw him on the sand far away from his usual 'rock'. He normally looks flat, and he is clear in color, so he blends with whatever is under him. he does have a brown cell like pattern on him that is very faint. last night he didn't move when I hit him with the flashlight. I thought it was dead, I scooped it up and it began to move, it then dropped a snail shell...it looked like it may have eaten the snail. Which would explain why most of my blue foots have been inhabiting the nassarius shells...I thought they were killing for housing despite the numerous empty larger shells...i even have a blue foot who inhabited a bumble bee snails shell!
Anyway...here's the pic: what is it?
BTW, I'm pretty sure that's digested snail coming out of the end of..."it"

 

[yosemite sam]

Looks like a polyclad flatworm. Don't put it back in your tank, they are predatory and it's probably the thing thats been eating at least some of your snails.

 

[natclanwy]

I can't offer any help on the ID, but if you have Bumble Bee snails they are your culprite for your snail losses. Bumble bees are whelks, all whelks are preditors and will hunt and kill other snails.