I have a 72" tank that's 18" deep. My lights are two blue HO actinics 36", two white HO 36" (both sets are T5), and two T8 36" blue actinics.

I do not believe that her foot is damaged, it appears to be uniformly brown with minor striations.
Could be water flow in the area that I originally placed her. The annoyance can certainly be the p-shrimp. Those fellas are the most opportunistic little animals that I've ever seen. If I were able to catch them I would remove them to the fuge.

The long tentacle has been with me going on two weeks. Didn't know about burying her foot. But I've definitely given her the opportunity to bury it herself. Sand bed is anywhere from an inch to three inches in parts. Safe mean would be right at the 2" level.