My Redy Eye Tetra has a red "cold sore" lookin on the top lip - is it a fungus or a wound?

My tank is 10gallon of peaceful community fish- and all of the sudden my red eye tetra has this spot- almost like a cold sore or a like her top lip is hanging off?? None of the other fish seem to have this, and I'm not sure what to do next? Do I take it out and isolate it until I figure it out? If I put her alone in a bowl she's likely get stressed and die anyway. please help! I just noticed it today, it was not there yesterday and I watch my fish very closely daily!