Hi!
Milky pees are the natural way of the body to excrete excess calcium. If it is just a one off, then it is not a problem whatsoever. You just have fed something rather high in calcium recently.
If you see lots of them on a regular basis or find dried white powder puddles, I would recommend to revise your diet as too much calcium/the wrong balance of calcium in the diet can lead to bladder stones in the longer term. Most people are not aware that most calcium in the diet comes via unfiltered water and the pellets (both amount - it should be no more than 1 tablespoon per piggy per day) and the fact that even no added calcium pellets are still higher in calcium that the veg highest in calcium). Please be careful with high calcium veg, too. It is always a balance act as you can't cut out all calcium from a diet without causing long term health issues that are every bit as bad as too much calcium in the diet.
Too little calcium in the diet can conversely also lead to lots of calcium pees.
Please take the time to read our diet guide, which looks at all food groups in details and the diet as a whole - and doesn't just concentrate on the veg while ignoring all foods that are not part of a human diet. Be aware that any changes will take several weeks to percolate through. the body will still have to work out any excess calcium.
Long Term Balanced General And Special Needs Guinea Pig Diets
If there are gritty crystals in the dried powdery calcium patches, then you need to see a vet for potential bladder stones/sludge.
I hope that this helps you?