Sounds like a dead pixel...
Since you are probably using Loki, your warranty is not void, as you have not unlocked the boot loader.
I'm not sure if LG would cover a dead pixel in their warranty (you would probably have to send it to them, since you are passed your 2 weeks)
I would recommend researching your options, and see if you can get a replacement from LG or maybe still from wherever you bought it.
You CAN restore to factory defaults, so that is not an issue you should be worried about... Just search xda a bit for how