I have said before, it is not open. It may have stayed that way for three weeks, but that's an indication that it is not happy. If it is a sun coral as others have thought, that would explain why it's closed up. Sun corals need to be placed in the shade and won't open up when they are in the light because they are non-photosynthetic. Try moving it to the bottom of the tank, in a shaded area.
Again, it is not opened. If that's how it has always been, then it is not a happy coral. Its polyps are not extended. We cannot say for sure what it is until it opens up.
You think this is what it's supposed to look like. Trust me -- it's not supposed to look like that all the time. It's not open. And we can't tell you what it is when it's closed like that. So put it in the shade and wait for it to open.