It appears to be a form of Euphyllia reproduction. If they don't like an area where they are they usually drop or as it appears to be melting off into the water.
Looking at your pictures I'd guess that you have the coral in a fairly strong current area and up off the bottom which gives it stronger light. If so it would be happier if you moved it to a lower light area or on the sand bed and in a lower flow area. They like slow gentle flow. If you give them this the polyp extension will surprise you. They can get HUGE.
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