Help with Unknown Predator

[Green Phantom]

Today, I looked into my tank and noticed that one of my goldenhead sleeper goby was dead. After pulling the body from the tank, I noticed that parts of its tail section was gone. I looked at my other sleeper goby and noticed there was a red bruise around its tail. Upon looking at the crevices between the live rock, I noticed a wormlike tentacle sticking out of the sand. As soon as it saw the light, it disappeared into the sand. Just wondering if anyone knows what kind of predator I have and how to get rid of it. This might explain what happened to my 2 Bangaii Cardinal Fish about two weeks ago.

Now I'm worried about the other sleeper goby. Help anyone??!! Kenny