I bought this 'rock' a few weeks ago. I always figured it was just a solid hard piece of coral but today I noticed the white moving in and out slightly with several tiny holes that open and close at will. What is it?
That's so sad, poor thing
If you want to try and save it, you can feed it. Keep your water parameters as perfect as possible and drop some Mysis bits onto the fleshy part after lights-out. The holes in the middle are mouths, if it's not too far gone they'll open up and accept the food. If you've got shrimp and fish that will steal the food, try placing the top 1/3 of a soda bottle over top of the coral to protect it for an hour or so while it eats.
I've got a very sick little hitchhiker that's slowly coming back with careful spot feeding, it can be done but it takes patience. If you can feed it for a few months (daily if possible), you could be rewarded with something astoundingly beautiful. Once it's better, which could take a year, you'll need to feed it a few times a week.