Here is a pretty good explanation on some of the reasons that coral bleach.
"Coral bleaching is a very common problem encountered in a home aquarium with the stony corals being much more susceptible to this process then soft corals. Bleaching is defined as a mass expulsion of the zooxznthellae or pigments from the coral, which typically appears white or translucent or only slightly pigmented tissue. A bleached coral is also a very much alive coral and all steps should be taken to help it to repair its population of zooanthellae to it tissues. While a lot of reef keepers do see a remarkable difference in growth rate using certain additives in no way does the lack of using these additives cause bleaching to occur. If all your water parameters check out and you do not have a tank that is heavily stocked with corals and you perform a partial water change at least once a month then your trace element levels should all be good enough to keep this coral healthy. If your tank is overstocked with corals and under maintained then these elements may be getting depleted to where an extremely slow growth rate or slow deterioration(in sever cases) may be noticed. Bleaching has many other causes. First and foremost and also the most common cause of bleaching in the aquarium is too much red spectrum of light, not enough of the right spectrum of light or not enough lighting over the coral period. This is a simple fix by simply upgrading your lighting system. The next most common problems are bacteria, chemicals, fungus, heavy metals, too high of a temperature, too low of a temperature, hypersalinity, hyposalinity, lack of water movement, medications, starvation, irritations by other corals or anemones in the tank, and stress. The key to fixing the coral is by finding out the cause of the bleaching. This may take some detective work. Fist check that the corals that are in close proximity to your brain coral are safe to be even in the same tank as the coral or that there are no more toxic corals downstream from your brain coral possibly causing any chemical warfare to be going on in your tank. If you have any anemones be sure that they are not only not in direct contact with this coral but also not close enough that the mucus produced by the anemone can come in contact with the coral. Some brain corals are extremely sensitive to the presence of any of the xenia species and should never be kept in the confines of tank together. Without knowing exactly what kind of brain coral you have, weather it is a favia, symphyllia, leporia, platygyra, oulophyllia, trachyphyllia or lobophyllia(there may be some I missed) it is hard for me to say exactly what your coral is lacking. Are you feeding your coral at all? Some of these corals have very long sweeper tentacles which are used to grab food and must need regular feeding to get the nutrition they require. Some brain corals do not require very intense light and can actually go through shock if not slowly introduced to higher light levels then what it is used to. There are many different reasons for a coral to start to bleach. Not using additives is not one of them. I hope this at least gives you a place to start."