I have a very small scratch/crack in my Air 2 screen

I only use my iPad at home for online schooling as a reader and sometimes very light gaming. I have always had a screen protector on it as well as an Apple smart case. I baby my iPad and have never once dropped or jarred it. I just bought a keyboard case for it and after it arrived, I took the iPad out of the case and removed the screen protector as I was going to replace it before putting it into the keyboard case (Zagg Rugged Book), when I too the screen protector off, I noticed something that resembled a hair and when I went to wipe it, it felt like either a deep scratch or a crack, my fingernail will catch on it. I am worried it may get worse.

Is there any way to tell the difference between a scratch and a crack? I really don't see how a scratch could have happened, but I won't rule it out either. Are these screens prone to scratch easily, or crack easily. Would putting another screen protector on it cause this? I do not notice it when I'm using he iPad and only when the screen in that area is black. I do not have AppleCare as it was a gift and I was unaware of AppleCare until it was too late. I am also 6 months into my one year warranty. I do realize that the one year does not cover cracks or scratches.

Am I reading the repair cost correctly, it's $379 or a replacement unit, mine is 128g, is that the same price, regardless of storage size? I guess for now I will keep an eye on it and hopefully it doesn't get and longer, it's about a half inch right now, the scratch/crack is right to left holding the iPad with the home button on the bottom. Thanks in advance for any help. I feel as though I can't stop obsessing over this, maybe because I love and use my iPad so much, thanks.