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And how does it effect value?

Typically Maples get milk spotting more than others do. I believe I read that it has to do with the cooling process the mint uses before the blanking process.
Myself, I would rather not see the milk spots so yes it may affect the price, but also the liquidity of them compared to others. The great thing about maples is the .9999 investment grade silver or gold that's in them. To get silver to this investment grade it needs a great deal of refining.

I forget the name of it, but I think it has to do with the chemicals used to clean the dies leaving residue if not properly rinsed. It doesn't show up until the coin goes into a humid environment (such as the US, but not Canada), like invisible ink. Best to put them into a capsule as soon as you pop the tube. The most non-invasive way to clean milk spots would be soaking them in household ammonia cleaner. If you don't care about grading, jewelry wipes work too but can leave very tiny scratches/scuffs, or you could use a pencil eraser.

This question has been asked around, nobody really has a good answer, except don't buy them, but go out to the grading services and check the populations, not many maples have been graded so maybe it just does not matter. They hold their value pretty well otherwise, but some people really do not like the milk spots.

lol. i saw the title of the topic and assumed the question was for the spot price for cows milk. one ounce perhaps?

I actually dont mind the milk spots so much. Lower premiums on government .999 is nice but also because it is almost like the patina of an older coin. Perhaps Im wrong but I just dont think you could fake it well.