Milky pee is excess calcium being excreted normally. It is advisable not to feed foods high in calcium too frequently, especially spinach, kale, dandelion.
Do not feed alfalfa/lucerne hay unless your piggies are under 4 months old, and even then never more than a handful in addition to their normal unlimited hay. it is very high in calcium and protein and not advisable for any but pregnant and nursing guinea pigs.
Please keep up the vitamin C - that is vital! Foods high in vitamin C are (sweet) peppers of any colour, broccoli, fresh coriander/cilantro, parsley, dill, mint or basil; feed a bit of these daily in their veg mix and vary between them.
Check powdery calcium patches for gritty bits - that could indicate bladder problems (crystals); plain powder is nothing to worry about. Guinea pigs have naturally a very alcaline urine and are more prone to bladder stone/sludge and bladder infection (UTI) problems than other animals.