Dry crispy skin on sourdough starter: is it mold? My sourdough starter is 2 months old. Some week or two ago it started to have a crispy skin on the top. The smell changed a bit, but it is still sweet-acid smell. I cover a jar with the starter with a kitchen paper towel. I replaced the jar with a clean one but the skin persists.

The bread I got from it is still decent.

Is it 
mold?
I would guess it is just drying out.  I keep my covered.  If it is on the counter preparing to be used, it is in a jar with a loose lid.  If it is the refrigerator, for longer intervals between baking, it is in a sealed plastic pint container.