mushroom coral have white rims

[IslandReefer]

Just my opinion, and assuming every other parameter is OK.
White rim around shrooms and xenia instant death....off hand it sounds like pH. I had similar problems in my small tank untill I increased the flow (added AQ500) (decrease the CO2 therefore inc. pH) and increased my Alk to 10+ (better buffering).
Soooo do a GOOD water change, tune up your flow and skimmer (if included), test and adjust ALK. Also check your house for other CO2 sources that could skew your pH (ie fire places ,closed windows etc)....moniter day vs. night change in pH....as an aside moniter temp days/ night, while your at it (ya never know)
A good place to start....my 2 cents worth,
Good Luck