It may be the case with some mobile air conditioners that you see something sitting there that looks like Styrofoam and want to remove it. Don’t do that, it’s there for a reason.
This material is hard compressed plastic, and thus can resemble Styrofoam. It is used in all mobile air conditioners to create an insulating drum. This makes the noise made by the unit less audible and therefore you are less bothered by it. The material does not adversely affect the further operation of the air conditioner.
The pressed plastic is finished with a white casing to minimise its visibility.