Infection? Tang aggresion. My Yellow Tang sometimes goes at my Foxface. Could be a parasite?
I do this super food paste thing that works for me in helping put weight on the fish. Sort of a home-made Masstick concoction. But it's nutritious and its a technique that helps get meds into the animals gut.
Food concoction (might seem expensive --- food and QT isn't necessarily cheap, probably why LFSs don't QT.)
Go to your petstore or
amazon and order Hikari Seaweed extreme
(
https://www.amazon.com/dp/B006JG2SPY/ref=cm_sw_r_cp_api_batCBb2X3AX42)
and Piscine Energetics 1MM PE Mysis Pellets (
https://www.amazon.com/dp/B01M5D5QJJ/ref=cm_sw_r_cp_api_x-sCBbDZ18EF4)
And pick up Seachem's metroplex, Seachem Focus and Garlic Guard (any garlic extract for fish would work, I just use this one)...
Amazon has a full package deal that includes Kanaplex. (Ultimate Seachem Aquarium Treatment Bundle Pack - Metroplex, Kanaplex, Focus, Garlic Guard - All In One Aquarium Treatment
https://www.amazon.com/dp/B07CN3KT89/ref=cm_sw_r_cp_api_RetCBbW0Z2XAX)
You're also going to get Selcon vitamin supplement (American Marine Selcon 60 mL
https://www.amazon.com/dp/B000255MGU/ref=cm_sw_r_cp_api_VdtCBb48ZXKZH)
Get a strong zip lock bag and pour some of each type of pellet into it. Then squirt in one dose of the selcon to help immune system and garlic guard for flavoring. Add 10-20ml of RO or distilled water -- whatever you've got. Mash it all up in the ziplock until it gets mushy. Then add in the metroplex and Focus meds. Mash it all up into a solid paste. You should be able to almost roll this paste into a ball. Put it in your fridge and let it sit for a couple of hours before using. You'll be able to take out a chunk and roll it into a ball with your fingers. Either mash it against the glass of your aquarium or mash it onto a small piece of dry rock and place on your aquarium floor bed for the animals to pick at. If your foxface is in QT you'll be able to more easily see her eat.
Even if she's in QT I would probably still feed this to my DT fish to help them fight off any parasites they might have picked up. Just make sure the paste is a good consistency that it sticks easily to your glass. Don't use too much. This bag should last you a couple of weeks in the fridge.
But doesn't sound like you've seen stringy poop or anything. She's definitly small, so it could be stress.
Is this a fowlr tank? Maybe isolate her in an acclimation box, just for observation to see if she eats. That would be easier than a full QT as well giving her a chance to eat and rest peacefully in case she's getting bullied. I'd still make her that food concoction, even without the meds. My latest version adds in Neptune's Cross-Over pellets, some ReefRoids and I've starting adding a probiotic in the mix. But I'm looking at getting them fat. And the feeding response has been good. It could help her put on weight and build her immune system.