Are you talking about sun coral? Sun coral open and close conditions are different from most other corals because they don't need light(not to confuse with "scared of light" which many members thought). Usually they open when they "smell" food in the water in disregard to the lighting condition.
From personal observations (not proven) they seems to have memory of when food is available. For example, if you feed them usually at 6:00pm for 2 months, they tend to open around that time. And if you change your feeding schedule to 1:00pm in the next 3 months, they tends to open around 1:00pm. I have some colonies open almost whole day
i have 2 polyps of black sun coral (now there is 3 a little one just opened up). well they started opening up and staying open all night b/c i feed them late i just have a question on how much each polyp should get and i need to get a better quirt thing then the one that comes with a refractometer lol.
How old is the tank. Any ammonia is bad. It must be zero. Either you have a real problem or you test kit is off. I once had a kit that registered .25 even when it was zero. Water chaanges always help dilute problems sso get that ammonia down. That is a lot bigger problem then closed polyps.