A few possibilities I can think of:
1. Examine the LED or CMOS module closely. It may have come loose from miniscope or might be on an angle.
2. Hold the miniscope in your hand, thumb covering the bottom, and LED high enough that you can see the artifact. Gently shake the miniscope in your hand and/or gently rotate the miniscope in your hand upside down and right side up. Does the artifact move/change at all with your hand movements? If yes, it's possible something is loose inside. Maybe one of the square dichroic mirrors (or even half ball lens).
Could possibly be something else, but those are two common issues I encounter when I notice strange artifacts in the FOV. Hope it helps!