as said it might not come out, plastic window's are not as maintenance free as they say, some dirt, as in the picture does penetrate into the plastic, you can try rubbing it out, or sanding it out [not worth it unless you really know what you are doing] then polishing it back to a shine, depends how deep it goes, its fly poop, the dirty little buggers!!! Cr** half their body weight
omg what a little factoid there, geek momment!!