You quickly damage your skin because it has not had a chance to build up the necessary pigments to protect itself. The same can occur with corals. The chloroplasts in the zooxanthellae of corals behave in a similar manner to those found in plants. When faced with lower light levels they produce more chlorophyll and when faced with too much light they will reduce the level of chlorophyll. Plants growing under the canopy of a forest have broader, darker green leaves than the same species of plant growing in the full sunshine.
Corals are no different.
If exposed to lower light levels they become darker brown in colour and their growth form becomes more elongated and tabular in an attempt to increase their surface area. Another potential problem could be oxygen poisoning. When deeper water corals are placed under brighter light, their zooxanthellae produce greater amounts of oxygen. Similar behaviour may exist in corals. When placing newly acquired corals into a HQI lit tank, they should be put in the lower regions away from direct light.