Need help with a hairy mushroom coral

I inherited a colony of hairy mushroom's last Tuesday. The colony is
comprised of 10 or 12 individuals (it's hard to tell, they're all grown
together) on a 3"x5" flat rock. I've placed the rock in the substrate at
the bottom of my 65g tank (24" height to 150w MH and 2x96w actinics).

They seemed to be doing well until yesterday when the "mouth" opening on
one started showing white calcification around the rim, which I figured
was just sediment stuck on the coral. Today the amount of white area has
spread outward and now covers a goodly amount (maybe 20%) of the
original mushroom. The "mouth" opening is now also twice as large as
normal and ragged edged (again, the mushroom's flesh has turned all
pasty white and appears almost like chalk).

I'm concerned that this one may be dead/and or dieing and whatever has
killed it could be contagious to the remaining mushrooms on that rock
and or other corals in the tank.

Any idea what this could be, and what I should be doing about it...

tia,

mark h