I know this is old but my golden head sleeper goby did the same thing and killed my plate coral. It was rather shocking to me as it didn't appear to be much sand but it went down hill quickly. This has been quite some time ago, 6 months or so. I left it in as it was difficult to get to and let it become part of the aquascaping. I had a guy come over one day to pick up some corals and he noticed the skeleton. He asked if it had babies when it died. I said, no, not that I'm aware of. Like I said, it's been a while, long enough that the structure is coated with coralline algae. But last week i noticed something and it turns out that there are at least 10 baby plates that are growing on the outside of the old skeleton. This is the second time that something that appears to have died came back from what was definitely dead. Glad I didn't pull it out.