The dry layer may be unavoidable in some cases. The only reason we would absolutely recommend discarding a dry layer is if it has a lot of (fuzzy) mold growing on it. Otherwise, just go ahead and mix it back in. We do NOT recommend a solid lid, since this might prevent colonization by environmental microbes, though (of course) we don’t know the consequences.
If you live in a really dry climate (or in a cold climate and have the heat on full blast) you can use a little bit of extra water (up to ~¼ tsp) when you feed your starter.
Of course, the dry layer itself could potentially impact the ecology of the starter too. For example, Pichia kudravzevii likes oxygen so usually accumulates at the top of starters with O2 exposure (or in starters with lots of mixing). The dry layer could prevent environmental organisms/O2 from coming in. So if you do have a starter with this layer, just make note of it when you submit your data. So if you have a dry layer, dont worry about it too much, but please DO tell us about it in the “Extra notes” section when you fill in your data. This will give us an idea of how common this phenomenon seems to be.