Hi,
Often times, even after removing the screws, you will need to take a screw drivers or knife and then cut or pry open the cover....I often start at one point and then work away from there making sure that the cover comes loose as I go...
I think that what you will find is that the fan blade is actually breaking apart...the plastic gets old ad brittle and then it comes apart...
Finding that part for most window units is very difficult and sometimes impossible... they seem to make them as throw away units...not to be repaired...
You can try to search it and find it, but even if you think it is right, you may get it only to find that it is not the correct part... happened to me too many times...
Your best bet may be to just buy a new unit...but definitely try to repair first...
Here is a tip that wrote about how to buy heating and airconditioning parts online.
How to find Air Conditioning, Heating and Furnace parts online.
heatman101