It appears that some of the tissue is dying on the gorgonian. You might cut the damaged tissue off at an area just into the healthy living tissue to stop the spread. Remove the dead tissue area and use a coral dip to treat before returing it to the dislay tank or quarantine tank. Just a thought. If you have tissue death you will be able to smell a rotten egg smell in that area of the gorgonian. Gorgonains love lots of water flow for feeding and other purposes.