Has my iPhone security been compromised? I just had something really concerning happen to my iPhone 6s (iOS 11.3.1, I update my apps everyday, not jailbroken). 

I was on the YouTube app when the screen completely froze and the screen’s colors looked slightly off (specs of red and green, etc. Could still see the video, but it stopped playing).  My phone almost never freezes, and in particular it’s never had the screen issues I've just described. 

Does this sound like my security might have been compromised? If so, what should I do, and are there any tips to keep my phone protected afterward?
No, that doesn't sound like a security problem. As far as I know, there's never been a compromise of an iPhone as recent as the 6S whose iOS version is kept up-to-date. It's more likely to be a hardware or software glitch.