I've been looking and it could perhaps be that the bones are "blocking it", so I'm adjusting something there and will keep an eye out in my own playthroughs.
I would recommend leaving the screen and returning, and if the creature still hangs in the air, load an earlier save.
It is a rare bug but that's a couple of times, now.
Edit: That your DESCENDER is still pink in your screenshot is of interest. If you've killed it twice, it should appear RED; 4 times, black.
After 4 kills it will notify you about a WELL, and attack you relentlessly until you DESCEND THE WELL, which will be accessible.