something killing my green mushroom

[sw_df27]

I recently bought a green mushroom at my lfs as one of my first things to see if I could keep it alive along with some zoas it did really good for 2 weeks was about the size of a half dollar but it looks like something is eating away at it it is now the size of a dime and I'm afraid it won't last much longer all that's in my tank is 2 clowns, 1 royal gramma, 1 scooter blenny a red sea star and a condy nem which is on the opposite side of the tank then the mushroom other then the little bugs that live in the rock and crushed coral I don't know what it could be my zoas are doing great along with another coral I have

 

[Cooperdude916]

I recently bought a green mushroom at my lfs as one of my first things to see if I could keep it alive along with some zoas it did really good for 2 weeks was about the size of a half dollar but it looks like something is eating away at it it is now the size of a dime and I'm afraid it won't last much longer all that's in my tank is 2 clowns, 1 royal gramma, 1 scooter blenny a red sea star and a condy nem which is on the opposite side of the tank then the mushroom other then the little bugs that live in the rock and crushed coral I don't know what it could be my zoas are doing great along with another coral I have

My guess is that it's the condy nem but not quiet sure. What size tank is it. If you can get a pic of the little bugs I might be able to identify them.