Yellow tangs are expensive now due to the ban in hawaii unless you are able to get a philippine specimen which is rare. Sent from Singapore Reef Club mobile app
There could be various reasons. But one likely one is that it is bleaching due to excessive light. Gonio generally prefer low light conditions. From the picture, it seems to be placed at a location that's bright enough for SPS/Monti. Perhaps its too bright.
Would suggest to bring it to a more shaded spot.