My masks keep slipping down and I have to keep pulling them up over my nose.

You did good, obviously nothing of any importance, no you should not keep quiet speak up when your life is in danger. Keep calm and Carry on.

1. WTH are they thinking?
   Some guesses:
   "It's easier to breathe this way. Derp."
   "Booyah! Suck it, libs!"
   "They can make me wear a mask, but they can't make me wear it properly!"
   "Everybody knows germs only come out through your mouth."
   Or, most likely, "Derp-a derp, derpity derp."

2. Should you have kept quiet?
   I think (though I'm not certain) that etiquette makes an exception, during a pandemic, to the rule about not correcting strangers. So we can classify your comment as a public service, and we thank you.

Chances are they already knew. I see it pretty consistently myself, often from people that look like they should know better.

Thank you U did it not just for yourself but everyone else around. Several retail clerks I've seen don't cover their nouse and I have call them out. One of them covered their nose the other one just looked at me and ignored me. Both of them were at the same stores so I have a feeling it was a management issue. The one that covered his nose told me it is so uncomfortable to wear this thing for 12 hours. I told him I was so sorry he had to but many people's lives depended on it especially old people like me. He said because he was behind the plexiglass green he felt he had some leeway. I told him I agreed as long as there was no one directly next to him.

Good for you! And, yes, continue calling them out. I'm seeing them pretty regularly here, and assume they're the people who don't believe in masks, but have to wear one anyway (for whatever reason). And if they don't believe the science of masks, they probably aren't taking other precautions, either.

Don't stop correcting them, group of girls in supermarket one didn't keep mask up I stared at them she sidetracked and didn't go near me lol

People here in NYC are pretty good concerning masks and social distancing.. I've spoken up a few times when people approach me though.. You keep on speaking up!