Yes, it is the soft screen. I think there is a section in the manual on cleaning does and don'ts but I can't find it.
Okay, you clean the soft screen with baking soda mixed with a little water to make a soft but not runny paste. Apply it with a soft cloth. Remove it with a soft. slightly dampened cloth with just enough water on it to NOT run into the frame. Then use a soft cloth to dry it.
I checked mine by doing a tiny corner of the screen. It worked like a miracle. In fact, I then cleaned the glass screen of my old computer the same way, and it was like magic.