about://(null) message with suspicious text appearing from nowhere. What's going on?

Today I was using my iPhone 4S (running iOS 6.1.3) to listen to music. I was on the app musixmatch, and it was the only app running (I always close apps I don't need via the task manager out of habit). When the music was playing, I left my phone there. All of a sudden, this message box popped up, titled "about://(null)" and saying "Why you no play Candy Crush!!" and with the choice to tap OK or Cancel. Screen shot is below. (I hope my music taste doesn't prevent anyone from helping me, it's only the second time I'm listening to this album)

I'm honestly scared. I don't think this is coming from the musixmatch app. Is this a virus? At first I thought it could be a malicious profile that's installed on my phone, but the only profile I have installed is the Soluto profile (soluto.com) that I use myself for managing my device. People who know me know I hate candy crush which is a popular iOS game, which makes this even more suspicious. I pressed OK, nothing happened. I'm at home connected to my own protected wifi network. There aren't any tech savvy people who are using this network. But now I'm scared someone can actually look inside my phone. The text sounds like something written exclusively for me, but I haven't done it, I honestly don't know where this message comes from. It's neither a text message nor a notification, after clicking okay, it's nowhere to be read again. I almost never use my iphone for internet browsing, so it can't be something I downloaded. Should I restore it? Cause if no one can explain this I'm not only going to do that, but also switch to android. At least android has an antivirus/firewall which you can download from its app store.

TLDR? I got this weird popup message out of nowhere, I didn't do it, but it sounds suspicious, and I need to know what's going on.

Thank you very much in advance for your help

-DH