What is killing by brain?

My beautiful pink brain coral is dying, or is already dead and I have no idea why. A week ago it was fine now much of the tissue is receeded or gone altogether and there is algae growing on it. I've had the coral for about two months and it was fine until a week or so ago. It did get knocked off it's rock by a snail a couple weeks ago so I moved the snail to another tank. There is a hole in the front of it (see photo) and one of the mouths is like another gaping hole now and doesn't close. It has a second mouth that will still open partially. I am noticing that the algae growth on it is spreading and the hermit crabs are always crawling on it. Also one of my mushrooms has disappeared just today. The other one has stayed shrivled up for days.
About the tank: water quality is great and stable, 30 gallon, 130 watt PC lighting, livestock includes the some corals (brain, daisy, torch, bubble, mushroom), crabs, purple lobster, shrimp, and fish (two clowns and a wrasse). The torch, bubble, and daisy are doing great. It's just the brain and mushroom. The only thing that has changed in the last two weeks is the addition of more crabs.
Is there any hope? My mom bought the brain coral for me so I'm pretty upset about it.

What exactly are your water parameters?Great and stable dont really tell us anything.
Well need the reading for ammonia,nitrite,nitrate,salinity,PH,alkalinity,temp,and magnesium.
What are you supplimenting with,how much and how often?
How much flow do have in the tank?
And last but not least.What kind of filtration are you running?

Ammonia 0, nitrate 0, nitrite 0, phosphate 0, calcium 440, salinity 1.026, pH 8.3, alkalinity (total and borate) and magnesium are within noral range forgot the number but i checked it about 3 days ago. As for what gets added, we use a coralline supplement that has Sr and Mo in it about once a week. The tank has two powerheads and the return head from the sump for flow. For filtration, a sump with skimmer and phosguard.

What are your strontium and molybdenum levels..since you are dosing that?
Never dose anything you don't test.
What salt do you use?

what kind of lighting do you have?

James,He's got 130 watts of PCs over a 30gal.

Did you happen to notice anything unusal about the coral right before it started loosing flesh?

If the coral when knocked over fell on any other corals, i.e. mushrooms or close to, the coral was probably damaged or affected by the mushroom and vise versa. you have probably lost the brain. in a 30 gallon you have a lot going on with the type of corals you have mentioned. all you can do is wait it out, if the brain is deteriating extensively, you would be better to pull it. sorry to hear of your difficulty. good luck.