When was the last time you changed your bulbs? As in... lighting. Four months... that's about the "good life" for most flourescent tubes. And depending on your halides, that's nearing the useful stretch on some of those. Conversely, if you just upgraded your lighting, the increased intensity can cause your rock to bleach. It's more likely for it to bleach from increased intensity than it is from lessening intensity. Have you recently added or changed out carbon? The sudden clearing of the water clarity with carbon can cause your rock to get shocked by the sudden increase in light.
High calcium dictates having high alk
... but doesn't necessarily mean it is high. If you have say, Ca
at 500, but alk
at 12 dkh
, that's too low. I run high Ca
, but my alk
is a steady 14.5 dkh
as well. And I've never had a snow storm, my montiporas and clam grow like crazy, and my corralline has never had bleaching that wasn't linked to too much light too sudden. Also, if you do water changes that drop your water levels, and you have the lights on, you can cause bleaching from light overdose.
What are your nitrate, phosphate, and ammonia readings? Do you dose with iodine or other supplements? What brand salt do you use?