I think that’s just coenosteum (normal structural cavities) or where the corallite used to be before it grew more vertically and skeleton wall eroded where there was no more tissue; or was fractured during fragging.
thank god i was freaking out because i started searching up and it said the coral was dying or something like there’s a disease that recedes the skeleton
Generally it’s good to have a flesh band that is established further down from the head but I’ve gotten hammers and torches from vendors that are pretty high up the skeleton.
*I was going on the assumption this is a newer addition. But if it’s been in the tank awhile then a receding flesh band could be a concern. I have a couple healthy corals where the bottom skeleton looks like yours. I’ll take a few pictures later as examples.