Why do my Dead Sea salts get wet?, C commented: Something similar happens to me every time a make my foot bath salts, the whole mixture turns into wet mush. I use urea and ordinary coarse sea salt and some essential oils. I store the foot bath salt in a airtight container with the lid tightly on. This doesn't happen to the individual substances on their own or if I mix the essential oils with the sea salt. Only when they are all mixed together.
Urea is a humectant, something that draws water from the atmosphere to our skin. The urea in your product is drawing water from the air to the product, causing the salts to dissolve. (Read more about urea in this post.)