Actually, new tanks go through "pod explosions" from time to time. They generally gather up on the glass in the hundreds or thousands even and shortly after go away. My tanks go through it all the time. I would start working on getting the fish to eat prepared foods as soon as you can. I really can't see a mandarin doing well in such a new tank even if it has live rock. :-(
I have an ORA mandarin which is supposed to eat pellets but didn't. She didn't eat prepared frozen food at first either. Luckily my biocube was up for over 6 months so I had some pods
plus I have a fuge in the back of the tank to help further. It got her by the month or so it took for her to eat frozen.
It's a beautiful fish but really a gamble if it can last or not. I hope you have lots of luck! Do you have a picture?