Don't try to scrape the gunk off of the sides when it is dry ! Refill with water and allow the gunk to soak and soften , then clean off with i stiff brush , but not a wire brush ! A scrubbing brush , plastic or bristle , should do the job .
Don't use anything like washing up liquid in the water if animals can get at the water to drink .
Just allow the hard crust to soak and soften and then plenty of elbow grease should do the job .