Usually this means your gregorian is dieing. It's caused be the polyps not doing well. Often this can be because of damage during shipping, and sometimes because something attacked it or ate it. Often buttterflys or angels will chow down on them.
You didn't tell us anything about your system, so it's really hard to go beyond any thing but general ideas.
If you have good water quality and the damage was shipping, this may stop on it's own. However, a lot of times it's fatal to the gregorian. These are not the easiest corals to keep.