I normally see a polyp close up, and not open, when given too much light to fast or not acclimated properly to intense light.

Usually, if I see them added new, into a tank, they will open the first day or 2, but then close up, and will not open till they either acclimate or start to melt. This normally happens when the light intensity is just too much to bare, IME.

I mostly see polyps bleach when given more light than they can tollerate, for long periods of time, but not enough light to shock them into closing. Depending on the polyp and the light intensity, I have seen this happen at a slow rate, over the course of a few weeks, to very fast- like 5 days or so.

Another indicator I have seen, that is a flag that the coral is receiving too much light, or as much light as it might want to tolerate, is speckling. A polyp that is not normally speckled, that develops them with more light may be getting close to handling as much light as it is going to be able- before bleaching or closing up. Not saying it will bleach/close, but the lighting is starting to push its limits. I have seen lots of cool morphs this way, Particularly in the PE's.