Lol, those are the feeding arms! All LPS and SPS coral will have some variation of them. They're lined with stinging cells (not strong enough to hurt you or even fish for that matter), corals use them to catch food out of the water column and you'll see them whenever you add food or at night or whenever because acans are always hungry! You can give them pieces of mysis shrimp to feed them, but seeing them is a sign that the coral is happy
Feeding tentacles like everyone else said. I know mine do this every night or once the lights go out. Really cool to watch, especially if you have the ability to take long videos after feeding. Then speed the video up to watch the coral eat.