Hi Jax, I suspect the hard crust just indicates that the surface of the starter dried out. I'd recommend covering your container more securely with plastic wrap so that your starter remains moist throughout. While we often say to cover your starter "loosely," this isn't because your starter needs exposure to air in order to ferment properly, but because the lid can blow off a tightly lidded starter if fermentation gases are allowed to build up. Plastic wrap will flex and shouldn't cause any issues. It doesn't sound like any harm was done if your starter was bubbling nicely beneath the crust, so there's no need for concern. Barb

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