5"+ black millipede like creature

I seem to have a huge hitchhiker that has finally revealed itself. I've seen at least 5" of its black body, covered in legs, and 5 white/black striped antennae. Its making a meal out of my pipe organ coral, so I was wondering if anyone knows what this is and how to get rid of it. Its too fast to pick up, or even get a picture of. Everytime I see it, he snaps back into the rocks. Please help!