Suddenly hiding anthia

[Coasting]

Sounds like something is stressing it.... Not necessarily that it's sick. But stress can sure lead to illness.
Perhaps try the mirror trick and distract the other fish to change up the environment a bit.
Add more anthias?

My male Lyretail did this when I switched to the new tank last month, and the Melanurus went on a 2 day attack spree against the Butterfly, and totally spazzed around the rest of the tank. He (anthias) used to be the boss on the tank and then all of a sudden I couldn't find him. Every so often he would move spots and I realised he was hiding in the rocks breathing slightly heavier/looking stressed. I tried using the mirror to keep the melanurus in 1 area and it didn't change anything with the anthias, he refused to come out of the rock. I wasn't even seeing him come out when I fed the tank.
I did then add a group of 6 red fairys and by the next day the lyretail was back out swimming and joining in with the group again. Everythings back to normal now.