what's eating my Acans

Red Linckia... usually considered reef safe. Thought to feed primarily on biofilm (bacteria). While they may scavenge on dead tissue, they've never been known to attack live, healthy corals.

 

Yeah red linkia was my second guess, though red's are usually more rare as apposed to orange linkia's. More than likely not the linkia.

Are there any other inhabitants in the tank? Any type of crabs? hermits, emeralds sally light foots...?

 

Might be time to set up a video camera to monitor your tank. IMOPE, most anything could be the culprit, especially if it's dietary needs are not being met. Furthermore, many marine organisms have a planktonic larval stage and are very minute when they settle out of the mix and begin to morph. While somewhat rare, I have seen a few cases where something (crabs, shrimp, lobsters, and even one urchin) mysteriously appeared between 6 months and 2 years after the last rock or coral was added.

 

It looks as if it's receding. Any other corals in close proximity? I've seen some of my corals with 6" plus sweeper tentacles do major damage.

 

The pink exposed skeleton is typically a sign of a bacterial infection.

The starfish should be fine, although it's likely an Echinaster species rather than a Linckia.

 

I'm curious as to what your water parameters are...

 

Detailed water parameters would definitely help. I don't think the acans are being eaten.