Just to chime in on this....I got a small red acan as one of my first coral and over two nights 2/3 of it dissappeared, I thought it was melting due to something in my tank but all parameters were normal.
Got a rainbow acan at a frag swap and it seemed to be doing fine the first day, then Im sitting infront of my tank just watching things and the bigger fo my 2 peppermint shrimp just walks out and starts going crazy at the mouth section of the acan....never expected a peppermint to do that....and they have tons of other stuff to eat in the tank. Scared him away with a spoon and move the acan up one level of rock and it has been left alone since but I am not sure if it is too late or not.
Setting up a QT this week in case I need to move them somewhere away from the shrimp for a while. Or I may fry up the shrimp and feed it to my dogs if he keeps picking on things.