Hi Zoe, sorry for the delay in responding to your question! It sounds like your starter may be drying out on top, so if you're not covering the jar securely with plastic wrap or a lid, you might want to consider doing so. The reason we say to "cover loosely" is not because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded jar and cause the lid to pop off. As long as you jar has sufficient head room for gases to collect, and you open the jar every now and then to vent the gases, this shouldn't be a problem. If dryness isn't the issue, then it's possible you're seeing some type of mold, but that seems unlikely unless you've left the starter unfed for several days at room temperature. Ordinarily a starter that has been fed regularly and consistently for over two weeks should be fairly resistant to mold. Without know your exact feeding routine, it's difficult to advise, but I think it's probably safe to continue feeding, and look for this new behavior to disappear. If you think it might be mold, take a portion of healthier looking starter from the bottom of the jar, clean out your container and utensils very well, and feed twice a day at room temperature for a few days until the starter begins to rise predictably. I hope this helps! Let us know how it goes. 

Barb

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