Hi Helen, it's hard for me to say if what you're seeing is mold, but a well-maintained mature sourdough starter is typically quite resistant to mold and other invaders. If you happen to be feeding your starter somewhat erratically at room temperature (with extended time between feedings in which the starter has fallen completely), and/or if your starter never quite developed to full maturity, it may be more prone to invasion. I'm always reluctant to advise throwing out an otherwise healthy-looking and performing starter, especially if the presence of mold is uncertain, so consider this an opportunity to step up your feeding routine and see if your starter recovers with no further signs of mold. I would remove the top portion of your starter and feed the lower portion (using a new jar or bowl) and feed twice a day at room temperature for a few days. Aim to replenish (feed) your starter when it's at its peak of rising, or just beginning to fall. This keeps yeast activity high, and will benefit your overall starter health. If need be you can adjust the ratio of ingredients in your starter to slow down or speed up fermentation, so that your twice a day feedings line up more with the 12 hour mark. A relatively larger feeding (more water/flour as compared to starter fed) will slow down fermentation, while a smaller feeding speeds things up. Temperature also plays an important role in the rate of fermentation, so if it's very warm where you live, you might want to consider trying a 1:2:2 or even 1:3:3 ratio of starter:water:flour, by weight. I suspect that feeding your starter a little more attentively for the next few days may be all you need to bring it back to its normal self. Let us know how it goes! 

Barb

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