Try putting it in an area with less flow it doesn’t seem to be dying in my opinion since it’s still got tissue. Maybe higher up in the tank so the sand doesn’t bother the coral
Keeping them on the sandbed is fine, as long as the sand isn’t blowing all over it. Many people keep their hammers on the sandbed
It’s more so issues with either flow or water parameters that are going to be the issue.
As long as the flesh is attached to the skeleton and the coral isn’t showing it’s skeleton - then you’re fine