Leave the tank as is for a while, to let the beneficial bacteria colony increase to meet the current bio load. If the tank wasn't cycled, it's doing so now. Inverts are more sensitive than fish, so you should expect shrimp and snails to die off first, which is what happened. That rock you got from the LFS
...do you think it was brought in along with a tank buyout, and not right from the ocean? I'm just thinking of the copper thing. Maybe you could go out and purchase a copper test to be sure.
I don't see any picture. Try to repost it.