Looks like a bristeworm. They're just cleaning up the dying flesh, they're probably not the cause of the death. Post your water paramers and what LPS specifically are dying if you want help with how to fix the underlying issue.
Definitely not a Bristle worm. From what I found on line it looks more like a Dorvilleidae worm. It say's not harmless but I find them on my dying LPS. My parameters are SAl 1.026, Alk 7.7, Cal 440, Nit .25 ,Phos 1.3 Mag1440. I keep the tank a little dirty just because LPS doesn't require the pristine conditions.
It is challenging to separate parasite from cleanup sometimes. Actually often it is hard.
If I had to stereotype I'd say coral parasites are really small, very stealthy, eat at night. There are so many predators for coral parasites they have to be smart.
For a worm like that -- guilty or not -- a melanurus wrasse would torture/eat it IME.