“I bought a used guitar and it came with this plastic pouch which has a pick inside and this other thing. It’s too short to be an extra string.”
My guess is that it is an extra string for a different instrument (probably a ukulele). You mention that you bought the guitar used. If it was purchased from a music shop, they might have thrown that in so you had a pick and they didn’t really care about a random string. As a string instrument player, I have had plenty of little packets like that laying around.
No, it doesn't look like it's inside with the nail polish.
It's white and gold, not blue and black
I see what you did there.