Risk:
* once you are above 5% you don’t have a risk of food poisoning, you can still get bacterial infections as Acetobacter (vinegar)
* normal populations grow in waves as one carbon source is used up another organism comes in to eat what is left, normal room temperature fermentation is about a week. During this time there is active outgassing of CO2. Having CO2 (anaerobic) is another barrier to infection with organisms as Acetobacter requiring oxygen to grow. Anyway on day eight the risk of another organism finding and building populations is low.
* does the white stuff feel soft? greasy? hard? granular like salt crystals? I also taste to determine this. Again the risk of something bad is extremely low.
* can you get a close up photo? Can you describe it if you are close like trying to thread a needle?
* shimmer on the surface makes me wonder about oils or residual cleaners in the carboy
* many bacterial and mold infections create gums which make long stringy “snot like” cell colonies, can you mix it out, especially if you scoop the surface off, ,,, aerobic organisms tend to float like a mat
* treating with K metabisulphite prevents most issues
welcome to wine making talk
wine is a low risk food, with several barriers to infection as pH/ SO2/ alcohol flavor issues are the normal risk