Side view of the cube

what kind of coral is that pink one? is that hard to keep?

It's a Pink Nepthea, a type of leather coral. One of my most successful corals, started as a 1 1/2 inch frag back in April. It's at the highest point in my tank, so it gets medium to high light and medium water flow. It used to shrink and expand alot throughout the day. I believe it was part of it's growth process because it does not do it too much now that it is big. Never had a problem with it, very easy IMO.