Help! Did I kill my Toadstool?

[jennytheleopardgrouper]

Thanks for the help. Everything else in the tank is healthy. All my other coral are fully extended. The mushrooms are possibly the nicest I've seen them.

I added several fish about 2 weeks ago. I tested a few days ago and the ammonia and nitrite were not quite at zero so I did a 10 gallon water change. (Levels are always good, but with the new fish...)
Salinity is good, temp is good. Nitrates say they're at zero. (I may need a new test, they've never been above zero.)
Just changed the charcoal now, it was about due (thanks for the reminder). I did find a few parasitic snails in the other end of the tank a few weeks ago but removed them and haven't seen any since. I think they go for the zoos first and all my zoos are fine. As for the deterioration, it's only in the stalk and I don't think I can cut that away, anyone? My tank is very low on algae, so does anyone know why the coral was growing it? Thanks,