It was my cleaner shrimp that got stuck and died.
I thought he was molting too and i couldnt find him in his normal spots and just like yours he was very friendly then he just vanished. Finally i was lookin and I happen to see his antennas sticking out. I left him alone thinking he had a new hiding place and he set there for a full day. Didnt come out to eat or anything. So finally i tried to direct feed him and i seen he tried to go for the food but couldnt move. So I picked up the rock and stuck my finger in the hole and tried to get him to back out and he couldnt. So I had to pull him out by his tail. I saved him from the black hole but he didnt make it.