Texture pop-outs are basically the most common glitches in video games, especially in the GTA series (ahem ahem) Add GTA and put it on mobile and you have the perfect storm to create some bugs. This is what you've captured here.
Why is that even considered a secret room? It's evidence of the Ghost World more than anything else. And why didn't you try to go through it?
Mobile GTA seems to have glitches and breaks everywhere. It's the same on Vice City as it is on San Andreas, probably another reason I would never pay for it to be honest, can't say much for the PS4 emulated versions either lol.