Well from the pic, he looks healthy. When you purchased him, did he look like "pac-man"? Meaning the "cut" on the center right of the pic. Did he come this way from the LFS and what did she/he say about it/not?.
Where do you have him located in the tank: top, top half, half, etc.? What is your salinity level? What is your Magnesium levels? If these can be provided, then help could probably be provided better. Also, since the tank is not fully mature especially w/smaller nano's, even the hardiest of LPS such as Acan's can have a hard time adjusting to your tanks parameters.
Acan's prefer low light levels. This could be the issue. I know with some of the pics that i have seen w/some of the reefers here, I noticed that they have their Acan's on the bottom to bottom 2/3rds of the tank; its where I have mine in my tank.
Thanks!