I have also developed this problem. Come to find out through reading Amazon reviews that others have the same issue. It seems to be a plastic coating on the Glass piece in the projector that is being burned by the bulb. The amazon review is provided below.

Amazon Review:

Caution Maybe Fire Hazard over Long term use!

February 8, 2018

Color: Black(Classical)|Verified Purchase

Worked great for a while. A month or so after using the lens picked up a small amount of dust. Not too big but an annoyance. Now here is where it starts to get interesting. I started seeing a yellowing of the projected image. It started out really small at first and kept growing. Eventually, it became so bad I removed the projector and put up my old 3m projector back to use. I decided to take it apart to see if it was just a simple fix. Opening it up shocked me! One of the glass pieces has a piece of plastic on it that was completely burnt by the projector light. I'm talking bad like fire in your house bad. I know there is a fan in the system to keep it cool and I also verified that the fan was working properly as well. I suggest looking at another projector. Be careful if you have this projector and don't leave it running unattended!!!