something a little similar happened to me over a year ago,checked all my parameters ,all were fine .i stopped checking for ammonia a long time ago so i deciced to check ammonia and that was it .seems i disturbed a plenum that had been working excellent for years in my sump.at that point i isolated the tank from the sump with my many valves to contain it,and used a seachem product to start lowering the ammonia levels ,besides water changes.and got to save a lot of my livestock and corals.i now have a little seachem ammonia warning chart that sticks inside the tank also by seachem.sounds though that your problem is different.do you use a 2 part calcium- alkilinity product .years ago before i had a calcium reactor i had something more similar to what you are describing.it seems that i added the products too soon after not letting the alk dissolve enough before adding the calcium ,i was told the products came out of solution i had a snowy flaky substance all over my tank and corals.just trying to help ,hope you find the cause