OK, let me see if I have this right...they get "bleached" because when they are stressed or not in good conditions they loose their zoolanxthe....(whatever) stuff?
They expell their zooxanthellae due to stress, it is the algae that lives within their tissue, essential for providing them with proper nutrition. They bleach from stress, lack of light, poor water quality etc.
I like the statement Bob Fenner always makes, something to the affect of check out the livestock at the LFS. If it doesn't meet the parameters of a good quality in good shape critter you are cutting your chances at suscess. Know what you are going to purchase and what it should look like, be doing and be eating.