The only reason why a coral would loose its zooxanthellae quickly ( also called coral bleaching ) that I know of is if the water temp gets to warm. I do not think you lighting is a major issue. There is probably a number of things causing your coral to react this way. Corals are resilient and will try their best to adapt to your system. Remember your system is also still new and trying to find its rhythm, you will have some corals that don't make it during the first year or two. Just don't panic and react to hastily, just about everything you do to try and help one coral will effect every other coral you have.
I wish I had a answer for why your coral is acting this way, but we are all still learning about these amazing creatures and how to care for them. Thats what make this hobby so interesting, although very frustrating at times. I hope your coral makes it, and maybe you will find out what is going on and be able to help the next person.
