Take a deep breath and don't worry! It is just the natural way of getting rid of some excess calcium. You will find that the pee dries to a powdery white patch. Unless there is hard, sharp grit (i.e. crystals) in the patch, you don't have to worry.
However, if your piggies are making calcium patches on a daily basis, you may want to consider reducing the calcium content in their diet to not contribute to a long term build-up of bladder stones.
Recommendations For A Balanced General Guinea Pig Diet
Guinea pig body quirks