Diagnosis
That's likely a piece of dust or debris that has settled on the internal lens or, more seriously, on the surface of the DLP chip inside the sealed optical engine. It gets projected as a fixed shadow.
How to Fix It
First, rule out an external problem. 1) Turn off the projector. 2) Gently wipe the external projection lens with a clean, dry microfiber cloth (like for glasses). Use a lens blower brush first to remove any loose grit. See if the spot moves or changes. If it's on the outside lens, this will fix it. If the spot is still there, try this: gently tap the side of the projector near the lens while it's on and displaying a bright, solid color (like a white screen). Sometimes, very fine dust can be dislodged by vibration and fall away from the critical optical path.
If That Didn't Work
If the spot is small, sharp, and black, it's almost certainly dust on the DLP chip. If it's larger and blurry, it could be on another internal lens. Unfortunately, there is no safe user fix for this. The optical engine is sealed to protect the laser and precision optics. Trying to open it yourself is dangerous (due to the laser) and will instantly void your warranty.
Heads Up
This is a risk with any DLP projector. To minimize it, avoid moving the projector while it's on or still hot, as that can stir up internal dust. If the spot is very distracting, you need to contact LG support for a professional cleaning or repair under warranty. Don't let anyone tell you to use "projector cleaning mode"—this model doesn't have one.