Pigs naturally get rid of their excess calcium this way like piggies&buns said. It does not mean they are poorly but if it happens all the time it can cause problems!
Some of mine have had phases of doing this, some have never had properly white pee, and some who do did it a lot never got anything nasty like bladder stones
but some did, and that's what you will be watching out for. If pee is 'sludgy' it can slow the flow and this allows stones to form and makes them more prone to urinary tract infections (UTI). Some of my piggies (older ones than yours) have had to have these stones removed and that is not an easy or cheap procedure, and sadly not always successful.
Try and encourage them to drink more water to keep everything flushing through... maybe by adding extra bottles or bowls near the dry food/hay. I had success with one reluctant drinker by putting the bottle spout right near the hidey where she slept and I could hear her drinking in the night! Give veg a wash before you offer it to get that bit of extra water in. Here's a link to a UK map showing where the 'hard' water areas are - that's a measure of the calcium in the water:
Hard Water Map
I live in a very hard water area (the kettle is
always scummy and the taps are a b*tch to clean!) so I mainly give bottled water to my pigs - soft water produced in Scotland which has a calcium level of 3 rather than the 400ish of my tap water! If you go that route you can see on the label the Ca levels (bear in mind some bottled waters are popular because they have
high Ca... for vegans and those who want to increase their Ca intake so do check those labels). Saying that, if my water bottle leaks I'll refill with tap water because they need fresh water all the time - that's really important!
Like healthy poops, white pee is just something piggie owners get used to looking out for and monitoring - piggy-savvy vets can give you more advice ☺
(I should really quit with these long posts... I confess I do talk a lot but I didn't realise until joining this forum that it's the same with my typing!)