This is Acropora yongei, aka the Green Slimer. It's one of my favorites, and I've seen huge colonies in other reefs. However, I've never had much luck keeping it alive, which is nuts. I can keep all kinds of SPS, but the Slimer is my kryptonite. From time to time I try a new frag and it does well for a few months or so. This piece was a gift from my best friend, but it too has perished. Nearby corals overgrew it, shading it and stinging it. I should have interceded but at the time I was far too busy to tackle that little job. I will try again, there's no doubt.
Check out that fluffy polyps sticking out of each corallite. This particular SPS doesn't need to be fed directly, and it captures bits of food during the night period when the tank is dark and the fish are resting.
Below is another Bali "Green Slimer" I kept years ago. It gets its name from the fact that it slimes heavily when handled, making it tough to glue in place. I got a tiny frag, and 9 months later it was beautiful but suddenly succumbed to various algae issues and nearly died completely. I pretty much wrote it off, but didn't pull it out of the tank. It started to regrow tissue and then suddenly there were new branchlets about to form. Another five months later (pictured), this coral was 3 times the size it was with five times as many branches, and looked better and better. It was mounted with Super Glue Gel to a nylon bolt, and never bothered again. Google "pegging corals" for more details, or watch my video here: https://www.youtube.com/watch?v=LxNAeTRHtmU