Hi Natalie, I'm sorry I wasn't able to respond to your question in a more timely fashion! The formation you describe on your starter sounds similar to this case and also this one. In both these examples the yeast in the starter was nutrient-deprived and so responded by adapting in this way. If this does look like what's going on with your starter, it's harmless, but does show that your starter isn't getting the nutrients it needs. What caused this to happen in the first case is that the baker was feeding his starter from a Brita (filtered) pitcher that had algae growing on it, and the algae was robbing the starter yeast from necessary nitrogen. I would suggest feeding a healthier-looking portion of your starter twice a day at room temperature according to our maintenance routine for a few days and see if you can restore the normal appearance and rising behavior of your starter. Also, be sure that your water source is pure and hasn't been exposed to algae. 

I hope this helps! Let us know how it goes. 

Barb

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