Wierd green thing.

[FishGeek]

The otherday i noticed a weird green snail like thing on one of my snails. I left if for a few days but kept my eye on it. Yesterday the snail crawled out of it's shell and the green thing was still attached to it. So I pulled it off and threw it away. And I think the snail died. This thing looked like a tightly spiraled green shell but was soft to the touch. I recently bought two peices of live rock and I'm prettly sure it came in on that. Anybody have any ideas as to what it was? I thought it looked like a big parasite that fishes get.

Simon