Two things I know about, 1. could have gotten stung by another coral, did you have this acan near another large LPS? 2. some corals that are on the sand bed can pick up a coral eating bug.
1.Put acan near a flow of cosistant water flow, so that any dead coral flesh can be washed away, instead of contaminating the rest of the acan.
2. put acan above sand bed, like on top of a small rock.
Hope this helps! Good Luck!