Hi P, mold grows from the surface down because it needs oxygen, so you're unlikely to see it dispersed in different spots within the starter unless it has had the opportunity to grow downwards. While a mature starter that is fed regularly is quite resistant to mold, a starter that is still in the creation phase hasn't yet developed the defenses of a mature starter. If your starter creation process calls for leaving the starter out at room temperature for 24 hours or more without feeding, it can be helpful to give the starter a good stir to prevent any mold spores from taking hold on the surface. While it's hard to say exactly what these white formations are, if it happens again you can attach a photo of your starter at customercare@kingarthurbaking.com and we'll be glad to take a look and see if we can help diagnose the problem. 

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