be a part of the fauna found in with live rock. Some people find those detrivores (and scavengers and carnivores a pest and even try to trap them. They are also called fire worms and can inflict a nasty wound. There are freshwater bristleworms too, though they are usually in the muck at the bottom of bodies of water and missed by researchers. If that is what you have Angel, get it out using a turkey baster or something where you don't have to touch it.
If that critter is indeed a sea cucumber, it can "clean" your water. And as you note, if threatened it can turn it's stomach inside out. Certainly a toxic defensive weapon on the reef, which doesn't work in an aquarium!
I'm sorry to hear that the second treatment was so rough on the fish. Maybe that treatment should be at a greater interval from the first and of a shorter duration.
All the best!
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