Acans aren't that hard to bring back from the near-dead; I'm currently reviving a rainbow acan from a single polyp where I cracked the skeleton in half, right down the middle!
All of the advice given here has been pretty much spot on; acans are a particularly hungry coral, coming from an area of the reef with low flow (sometimes only ocurring with changing tides) and high turbidity, as organic sediments and food particles slowly wash through the water. For this reason, acans are all but indestructible if you put them in an area of low flow, indirect light, and give them access to food multiple times a day. Although they do respond well to spot-feedings of mysis and protein-rich pellets and dry foods, my favorite food for spurring on their growth is assuredly BRS Reef Chilli; it has a VERY fine particle size, and I'm able to add a lot of it many times over the course of a day without having any adverse affects on my water. When I really feed a ton of it, I have acans grow new heads just days after being fragged.
Regardless of which route you choose, lower flow, indirect light, and as much food as the acan will take should send it spiraling upwards and growing outwards pretty darned quickly.
Good luck!
- Ben