I had the same exact ones, yes they are considered good but I actually saw it at night going after a sleeping fish killing it. Hard to catch so I removed the rock placed in a 5g bucket with water, by accident left it outside during winter about 45 degrees, next morning I had about 9 dead ones in the bucket. Cold killed them. They are nasty.
It's a bristle worm. We always use tweezers to remove them because they release them in your skin and it is not fun.. they are a good scavenger but can easily get out of control if you feed heavily like we do. We keep them in check by taking out the ones we see during our weekly water change.
They are creepy as heck and can get pretty big and reproduce like crazy. I'll never use live rock to start a tank again. I used dead rock for my FOWLR and am quite happy. Good CUC or not, they wig me out. Search for bristle worms on YouTube and you'll find some frightening stuff.