I am quarantining this fish and a platy together, they are new fish.
Platy came from a trusted shop who quarantine for 2 weeks and treat all livebearers with esha 2000/exit and NT labs dewormer. Bristlenose is 2 inches and came from a friend whose tank is massively overstocked, she’s a baby from his tank.
I never think it's wise to treat unless the fishes are definitely ill (and the platy has just recently undergone medication). Do you know if that's the BN's normal colouring (there's always variation between fish) or if this has changed? [Even that might be difficult to tell, if she's a baby - it may be what becomes natural for her.] I would continue quarantining both the BN and the platy for a full month - longer if you're not sure what's happening. If the fish don't eat, or further symptoms develop, then it might be wise to treat with a broad-spectrum medication or with something more specific if it seems that's required.