Flow is everything.
This is what can make many corals like this one tricky for some. Poor flow can lead to gunk gathering in the ‘cup’ or other areas that form a ‘cup’ and basically can suffocate the coral of light.
This can happen when first placed or after many years of growth. A coral can easily grow on such a manner that it restricts flow to the rest of it.
This is most easily seen in spiny pectinia. Due to its growth pattern it is extremely difficult to keep proper flow to the entire colony.