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Corals don't have brains and so can't be trained to do anything, as such. They do have a preprogramed reaction to stimuli which you can use to your advantage.
For Tubastraea (and many other corals) high flow tends to stimulate extension of tentacles. Tomascik et al. (1997) report wild colonies of T. micranta being fully expanded in areas of high flow (greater than one metre per second). It is not something that I have seen (and I have seen many colonies on dives), but most of my dives have been in relatively calm water. I do have a colony in one of my tanks that is frequently expanded while the lights are on, but it gets high flow (it is at the intersection of two powerheads) which tends to support the observations of Tomascik et al. 45commando's observations are also consistent.
I think the main reason that corals such as Tubastraea don't expand more often in tanks is they don't get enough flow or they are starving (or both).
Tubastraea (and other asymbiotic corals) get ALL their energy from food. If they don't get enough food they won't have a lot of energy. Tentacle extension requires energy and so if energy levels are low, they are not going to waste energy at times when prey capture may be less likely. As their tends to be more prey items present at night than during the day, a starved coral is more likely not to expand during the day. A well fed coral will have plenty of energy and so may expand at other times of day in attempt to capture even more food.
I think the main reason people see their colonies expand when being fed at the same time each day is less about the coral "learning" and more about a coral having enough energy to expand more often, and also the stimulus of flow.
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ATJ
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