iMac will occasionally wake itself - weird blue screen

Hi Everyone, I am relatively new to Mac computers. I purchased an iMac last summer (27" 5K) and have noticed recently the computer will occasionally wake itself and display a neon blue screen. My login name/icon appears as an odd pink pixelated logo. The login screen goes back to normal when I hit the ESC key.

I can't pinpoint exactly what is causing this. It seems to be pretty random. A week may go by and it does not happen once, then it will all the sudden happen 2-3 times in the middle of the night. I spoke with Apple support and they recommended resetting the VRAM/PRAM, which I have just tried doing. Though I'm not sure if anything actually happened when I held the keys down. None of the settings I was warned would reset back to default settings actually changed.

Has anyone else experienced anything like this? I've attached a picture of the issue I am having.