Looks like it’s just receding and not dropping a head, however that’s still not all good news. Most of my euphyllia and bubble corals (similar skeletal/flesh structure) will do this when exposed to higher than preferred flow or something is aggravating the flesh.
Judging by the pics, I would move the coral to an area where it will be sheltered from flow and, if possible, an area where it can be viewed easily for monitoring throughout the day and night for potential pests.