mushroom coral have white rims

[jennytheleopardgrouper]

Hello all, just trolling for some info on my mushrooms. I have several, over several rocks and most have developed white outer rims. Something tells me this isnt a good thing, that they're stressed or being poisoned..anyone know? I checked my parameters yesterday....ammonia 0-0.05, and nitrites at 0. I didnt test nitrates, although they should also be quite low, as I tested recently and they were less than 5. I believe I've lost a snail, which may account for the small trace of ammonia, although the mushroom rims were white about 2 weeks before the demise of my snail. Thanks.

P.S My small xenia frag died suddenly the other morning-no warning-it was suddenly a small green lump...