Does this look like ich? I’m new to the hobby and didn’t quarantine, I know I know I will be in the future, this is the only livestock in the aquarium so I’m not too worried about infection, the white dots have mostly gone away, so I’m wondering if it’s the substrate.
Welcome to Reef2Reef!
I agree, all of those spots are sand/substrate sticking to the fish, with the exception of the spot above its eye. That *could* be ich, no way to tell for sure.
I would say that adding a mandarin as the first fish in a tank is usually not a good idea. Do you know, was it a Biota tank raised mandarin? if so, you might be able to get it to eat enough. If it was a wild caught mandarin, the tank won't have enough microfauna for it to graze on and you'll need to supplement with live pods.