First of all welcome to wine making talk. , , , where to start?
Wine is a preservative system! If the pH is below 4.0 and you have 10%+ alcohol and it is dry you will not have food poisoning organisms. Your main risk is that an aerobic (requires oxygen) organism is in the carboy. This type of microbe needs To ferment alternate carbon sources. , , , usually alcohol, , , , usually producing vinegar.
Diagnosis. If microbial in origin it should feel soft/ greasy. Appearance, you have some white colony shape and you have some floating film appearance. As Brian said smell (for vinegar/ off notes). If you had a microscope (or kids that are taking high school biology) you could pull a sample out and see if it is a cocci (oblong) shape and likely Acetobacter,, or long filaments likely a mold.
At this point I would take some paper towel and try to wipe it off. In a carboy I have taken a metal as knife or skewer rolled an inch strip of towel and held it with a rubber band for wiping. Microbial breaks up so be gentle. Repeat till clean. Then treat with a double dose of metabisulphite / 100ppm. This will be at the level where some folks will taste it but most won’t. Can you remove the head space? ex marbles?
In the future any time the carboy is opened treat the wine with 50ppm metabisulphite, and keep the head space at the minimum. Air/ oxidation is your big risk when you have a finished wine.