Green brittle stars are a danger to fish if not well fed. Never heard of them hurting corals.
The second pic actually looks like a nudibranch to me, but that's outside my area of expertise so I offer no guarantees. That crab, though... WTF? I've never seen a crab like that before. Cool. Also the most likely culprit.
Go to live aquaria or some other site and look up saltwater crabs. Then filter down to "Reef Safe" only and see how few are left, if any. Then read the descriptions and you'll probably see a warning about them eating stuff we don't want them to.
Take the Emerald crab, for example. Marketed as a solution to Valonia (bubble( algae. Herbivore, reef safe.... except that every one I've ever had has eaten coral, and the net is drowning in similar stories.
Next, we cover the Porcelain anemone crab. Nifty little guy. Almost defenseless, they hide in an anemone or LPS
coral. Mine hid in a torch coral. They are filter feeders, with two fan like structures they hold out in the current. But I watched mine put those away and try to cut the tail off a Nassarius snail on numerous occasions.
Bottom line, even reef safe crabs are not reef safe. Since you described polyps simply going missing, and finding one detached and on the sand, I'd say they are being eaten by the crab. Try trapping it and keeping it isolated. If you still lose polyps, I'll mail you a free mushroom coral. My money is on the crab.