Crust on starter
I've been using a starter off and on for a few years now and just this week, twice, I've seen it develop a hard crust on the top as opposed to a watery liquid that one might normally see.
The only thing I can pinpoint that was different was that one time it was in a rather small glass container, in the fridge where normally it would be one much larger (8 oz vs about 16). I always cover with just a coffee filter and rubber band. It had been about a week when I checked it and found the entire top layer was a hard crust.
The second time was when I was actively reviving it to use and was after a 3rd feeding. I had it in the oven with the light on overnight since it's getting chilly now. It was just lightly covered with a cloth to keep out any dust. The top had a hard look and feel to it though it had obviously grown well since the previous night's feeding and the part just below it was fine. I just went ahead and used it as is since Saturday is the only day I have enough time to do it but the resulting loaf didn't rise very well at all.
Why would it be developing the crust and is it unusable?
My guess is that you'd not have a crust if you covered your starter with an air-tight lid. However, since starter can be dried out, crumbled, and put into a plastic bag for air transport (thereby avoiding having to throw a jar of the prescious stuff into an TSA bin during a security check), I see no reason why you couldn't rehydrate the crust and continue feeding it as usual.