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it sounds like a calcium pee and it is nothing to worry about. Only if the deposit is very gritty do you need to worry, as that could indicate bladder sludge. Milky pee that dries into a fine powdery deposit is the way that a guinea pig body is getting rid of excess calcium. If you have a lot, I would start reducing the pellets (especially if you have alfalfa based pellets) and filter the water.