I have a sickly looking gbta I've had for about 3 weeks. Initially it acclimated good and looked fine for a few days. Then I thought it had gotten caught in an intake somewhere but later discovered it's missing tentacle was due to it biting them off.
Now The nem inflates randomly, deflates randomly. Yet keeps a good foot on the rock and even moves (always away from the light) He's also covered randomly by tanish slime on about 25% of his disk.
No pics, it's behind the rocks.
Leave him be, or treat with amoxicillin? And if treat. How much?
I don't know anything about metal halides so I can't tell you if your lighting is ok. How old is the tank? If you give it favorable conditions, it will recover. My bet would be immature tank or bad parameters.
30g Long Frag Tank
3g Pico Reef